Noble opal is a mineral with an iridescent color. Noble opal: photo, characteristics and meaning of the stone Noble black opal

Precious types of opal can be of a wide variety of colors, including: black, white, blue, purple (light and dark), green, red and even yellow.

Moreover, the play of color can be far from continuous, but dotted or mosaic. For this reason, there are a huge number of varieties of opal. The brilliance of the stone can also be different, for example, glass or wax. Although the hardness level of opal is above average, it is still brittle and can easily break if given enough pressure.

Often, opal is found in the form of streaks or dense masses that resemble glass. Sometimes the stone is found in the form of a stalactite. However, the fact remains unchanged that large deposits of this stone are found in volcanic areas, that is, where a volcano used to operate or is still active.

Opal deposits

It is not known exactly when opal was first discovered, but the largest amount of this stone was found in Australia. The largest opal in the world was also found there - its weight exceeded 5 kg, and its size was 23 × 12 cm.

Large deposits of opals were also found in Brazil. It was there that in 1998 a stone was found, the total mass of which exceeded 4300 grams. The stone was later valued at $60,000.

Other opal deposits: Mexico, Japan, Czech Republic, USA and a few small sources in Russia and Ukraine.

noble opal

Jewelers value first of all noble opals - almost all over the world it is customary to classify such stones as precious. They are distinguished by a bright pure color and iridescent tints. This feature is characteristic of translucent stones, which are quite rare. In total, the ranks of noble opals include six types:

1. White- has light blue tones, translucent.
2. black opal- has a purple, blue, green, burgundy hue.
3. fire opal- red or yellow stone.
4. Royal- multi-color drawing.
5. Girasol- colorless or with a bluish tint.
6. Lechos-opal- a completely green stone, with a play of all shades of this color.

synthetic opal

It is always worth remembering that whether you are presented with an Australian opal or a stone found in Japan, the beautiful brilliance and play of light may not be real, but the opal is synthetic.

However, do not immediately get upset: artificial opal can often be a good alternative to natural stone. Its obvious advantages are greater resistance to external influences and the absence of reaction to too low air humidity, from which natural opals suffer so much. Today, synthetic opals have learned to make really high-quality ones and their unusual overflows look very impressive. However, the difference in price can be truly enormous.

The magical and healing properties of opal

Opal stone, according to ancient beliefs, is able to support the talents of its owner, and it does not matter whether they are directed for good or for evil.

In addition, it is believed that opal helps with heart disease and protects against infectious diseases. In any case, opal has long been considered valuable, and often not because of its beauty, but precisely because of the properties that it possessed. For example, in ancient Rome, opal was considered a symbol of love.

Who suits opal

As for the distribution of opals according to the signs of the Zodiac: Leo, Sagittarius and Aries, as expected, fire opals are recommended, however, as well as emotional representatives of the Scorpio sign. The latter are also the only sign who can wear black opal, which is especially contraindicated for Taurus. Blue opals are suitable for Pisces.

However, it is still worth remembering that opal is a stone of spiritual and mental, and not material wealth. And first of all, people who are not mercantile and pursuing positive goals, as well as striving for self-development, should count on his help. Opal is well suited for creative individuals.

noble opal (Opal Noble) refers to an amorphous variety of quartz with a variable water content. Its feature is the absence of a crystalline structure, which distinguishes opal from other stones, including crystalline forms of silica, classified as minerals.

The spherical particles of silicon oxide in opal are packed into dense lattices. In noble opal, they are ordered, which provides a bewitching effect - opalescence. This specific ebb with iridescent overflow of colors can be either in one color scheme or in several. For maximum manifestation of opalescence, noble opal is treated with a cabochon. Strengthening the play of colors is also achieved by creating doublets and triplets, i.e. compound stones of two or three layers. For the lower one, a darker substrate of another material is used, and for the upper one, an opalescent layer of opal is used.

Varieties of noble opal are classified according to the color of the base material, the so-called body, without taking into account the play of colors. These include white opal (milky white); rare, hence more expensive black opal; harlequin (colorless or yellowish); fire opal (orange), girasol (blue or white transparent).

About 97% of noble opal is mined in Australia, where it was first discovered in the middle of the 19th century. Opal is the official stone symbol of Australia. Deposits are also being developed in Mexico, Brazil, USA, Ethiopia.

Since opal can lose water, it should not be stored in low humidity conditions, as this can lead to clouding and cracking of the stone. Jewelers advise wearing opal jewelry more often to avoid this. You can also periodically immerse products in clean water.

Opal group

Synonyms: Opalite, redundant name.

Opal - This ancient stone is an amorphous mineral. It is usually observed in the form of dense sinter formations of various shapes - crusts, kidney-shaped masses, oolites, earthy aggregates. It occurs in the form of many varieties that differ in water content and impurities. In addition, opal composes siliceous sedimentary rocks (diatomite, flask).

Fire Opal. Photo from the mineralogical museum

Opal Formula

SiO 2 nH 2 O

Origin of the name opal

It is believed that the name "opal" comes from the Sanskrit word "fell" (Upala), which means a precious stone, or gem.

Historical outline

Opal in ancient times was imported from India along with other less valuable stones delivered from Egypt, Arabia and other Mediterranean countries. However, from the time of Roman rule and almost until the XIX century. the main source of opals were deposits in Hungary. In Byzantine times, the main market for Hungarian opals was located in Constantinople. This stone (opalus, paederos) was very popular with the ancient Romans. Pliny describes precious opal of extraordinary beauty and tells the story of Nonius, a Roman senator who renounced his home and country, but did not give an opal ring the size of a walnut at the request of Mark Antony. It is known that the best opals were then valued at $ 20,000 or more at the modern rate. It has been highly regarded for a long time. De Bodt considered it the best gem, and Dutens (Dutens, 1779), who held the same opinion, wrote that the most beautiful opals were valued as highly as diamonds. Ure wrote: "In our time, beautiful medium-sized opals are often sold for the price of diamonds of the same size, and especially Muslims are reckless in determining the true value of these stones."
In the 19th century Opal's popularity began to decline, in part because of the stupid prejudice that the stone was considered unlucky (it is said that such a belief began after the appearance of Walter Scott's romantic novel Anna of Geirstein, or The Girl from Darkness, published in 1829, whose characters believed in the power of this stone). The high price of opals, especially milky noble opals from Hungary, also fell sharply at this time due to the import from Australia of a large number of fine, mainly black, opals. Black opals with patterned color patterns, such as harlequin opals, are currently one of the most valuable varieties of this stone. Black opals from an unidentified deposit were known in Europe as early as the 18th century. In addition, the very annoying property of opals to crack in the air, as well as the weakening of the play of colors and transparency when dehydrated or in contact with fats and oils, influenced the lowering of the price of opals. In this regard, it should be protected from sudden changes in temperature during the jewelry processing of stone. The property of opals to crack is very different for samples from different deposits and even for samples from the same deposit. It is believed that it is least pronounced in Hungarian opals and maximum in opals from Central America, Mexico and especially from Virgin Valley, Nevada.

Opal varieties

The nomenclature of opal varieties, like the nomenclature of fine-grained varieties of quartz, has a huge number of names identified mainly on the basis of color, external textural elements of the structure, state of aggregation, and other minor features. Only a few of these names are preserved as synonyms. Below we consider more clearly distinct varieties, ideas about which are widely used in the literature.

It is distinguished by an iridescent (opalescent) play of colors: different shades of blue, less often reddish. There are several varieties;

• Noble scorched precious opal showing play of colors
• Fiery opal - red to yellow with fire-like reflections
• Girasol - uniform bluish or reddish color

common opal

Ordinary opals generally include colorless and devoid of play of colors opals, the value of which as an ornamental material is determined by their transparency or beauty of the pattern. Ordinary opals also include milky opal, hyalite, opaque to translucent stones of other common colors, woody opal, rock-forming opal silica, etc.

Hyalite(or hyaline variety of opal). It is colorless and transparent as glass stone. It is often found in the form of crusts with a grape-shaped, spherical or kidney-shaped surface, as well as in the form of stalactites. Sometimes slightly colored, usually in blue, green, greenish-yellow or yellowish tones, there are gradual transitions to translucent milky or white varieties. Hyalite typically has a higher refractive index and lower water content than most opals. The name hyalite comes from the Greek word sakhos, meaning glass.

Hydrofan(Hydrophane). A white or light-colored translucent to opaque variety of opal that becomes almost transparent when immersed in water.

Tabashir(Tabasheer, tabashir). These are accumulations of milky white opal silica deposited in bamboo partitions. Its density is 0.5-0.6 and N is about 1.12. The dried material absorbs water very strongly and becomes transparent.

Cacholong(Cacholong). An opaque, porcelain-like, white to yellowish variety of opal with a pearly sheen; quite porous, but it does not become transparent after immersion in water.

Milk opal (Milk opal). Translucent to opaque varieties of common opal are milky white, pale bluish white or greenish white.

Resin opal, matt yellow with a resinous sheen.

Woody opal - pseudomorphosis (false form) on wood.

Mineral changes

Fluctuations in water content in various natural samples of opal, apparently associated with different conditions for the formation of these opals and with their later dehydration.

It occurs in the form of cortical formations with a grape-shaped, globular, reniform or wavy surface; in the form of stalactite-like or coral-like masses; in the form of concretions of a tuberous or irregular shape. Opal is usually confined to cavities and veinlets. It also forms siliceous tuffs and porous or massive geyserites; the structures of such formations are very diverse - fibrous, leafy, shelly, spongy (some accumulations of opal are outwardly similar to cauliflower), scaly-massive and massive. In addition, the stone acts as a rock-forming mineral, composing powerful strata of diatomites, loose or firmly cemented and dense.

Physical properties
Optical

fiery color

Mineral color. The purest are white and colorless transparent stones; usually there are milky-white or bluish-white opals. Depending on the presence of coloring impurities, they acquire a different color, usually pale shades: from yellow to yellowish-brown and brown, reddish-brown, green and blue, and also from gray to black. Sometimes opals are dyed red with finely scattered cinnabar or orange-yellow with orpiment. Opal also usually has a brown or reddish-brown color due to iron oxides. In some opals (noble opals) there is a rich internal play of colors in reflected light - opalescence.

The coloration is due to three reasons: pigmentation by finely dispersed foreign material, interference of light rays, in which the play of colors of the precious opal is observed, and scattering of light, which leads to a pale bluish tint in reflected light and a brownish tint in transmitted light. The cloudy milky white color of opals is due to the scattering of light. In any given specimen of opal, the coloration may be due to two or even all three of the above, but intense pigmentation with diffuse opaque foreign material can reduce the transmission of light so much that it becomes impossible to observe scattering or interference of light. In transparent and almost transparent types of precious opal, the play of colors is mainly due to the interference of light, although they sometimes show a faint color due to the admixture of a foreign coloring matter. For the hyaline variety of opal, neither coloring impurities of other substances, nor the phenomena of interference and light scattering are characteristic.

The appearance of a play of colors in a noble opal is not clear. This discoloration is observed in reflected light and is not visible or very weakly manifested in transmitted light. Individual colors are often characterized by significant spectral purity and intensity. For some red colors, a spectral line with a width of about 20 A was measured. Color play does not occur under monochromatic illumination. Although these colors are caused by the interference of light rays, the nature of the structure responsible for this effect remains problematic. Possibly, this structure represents an open regular spatial network of cristobalite crystallites. However, some samples of noble opals give powder X-ray diffraction patterns of a crystalline substance, while others are X-ray amorphous. It was also pointed out that the play of colors may be due to the presence of thin interlayers with a refractive index that differs from the refractive index of adjacent interlayers, as well as systems of cracks or slits, possibly made by opal with a slightly different refractive index. The strong saturation of colors suggests that the structure that causes their occurrence has a periodicity of considerable magnitude. It is possible that the play of opal colors is associated with mechanical tensions that occur during the drying of the original gel and opal.

• The color of the streak is white to faintly colored in the colored varieties.
• Glass luster. Wax finish, matte.
• Transparency Opals range in translucency from opaque to transparent, but mostly translucent and translucent stones are found.

Opal is isotropic, refractive indices and density vary with water content.

Mechanical properties

• Hardness 5.5-6.5.
• Quite brittle, sometimes noticeably brittle mineral. It can spontaneously crack with a sudden change in temperature and even with slight dehydration.
• The density of opals varies mainly depending on the water content, porosity and fissuring, usually from 1.99 to 2.25, but can drop to 1.8 in sintered and porous opals. Density increases with the absorption of water, especially in hydrophane and other highly porous dehydrated opals.
• Cleavage is absent.
• The fracture is conchoidal or flat-conchoidal to evenly sinuous and occasionally spongy, and in some opals from splintery to uneven.

Chemical properties

Synthesis. By reacting fluorosilicic acid with glass, dense, opal-like masses of hydrous silica were obtained, sometimes transparent or with a play of colors. Similar substances were obtained, in addition, by precipitation under various conditions of sodium or potassium silicate, ethyl silicate solutions, as well as by the decomposition of silicon chloride, silicon fluoride. Opal can also be obtained in other ways.

Other properties

Other properties: very brittle, cracks when dehydrated. The color of opal does not change when exposed to x-rays. Some opals, especially hyalite, fluoresce in greenish-yellow colors under ultraviolet light due to the presence of a small amount of uranyl ions in them.

Diagnostic signs

Characterized by a waxy or matte sheen, conchoidal fracture, dense structure. It differs from chalcedony by lower hardness and density, as well as brittleness. When heated in a test tube, the litter releases water.

Opal jewelry, practical application

Noble opals are precious stones. Earrings, beads, pendants, rhinestones are made from opals. Opal rocks are used in the construction industry, for the production of abrasive materials, for thermal insulation and other purposes.

Occurring terms as mixed mineral formations and structural varieties

Opal-agag (Opalachat) = banded opal

Opal-allophane (Opalallophan), redundant name = mixture of halloysite with variscite (the so-called "schrötterite")

Opaline (Opalm) = opal pseudomorph after serpentine

Opal jasper (Opaljaspis) = mixture of opal and jasper

Opal cat's eye (Opalkatzenauge) = opal with the inclusion of crocidolite fibers

Opal-onyx (Opalonyx), superfluous name = opal-agate

Mysterious opal is famous for its medicinal and mystical properties. The pebble is shrouded in legends among many peoples of the world, most of these legends are firmly associated with the images of gods and goddesses. The lovely structure of the stone creates many colors for it: there are fiery red, milky white, blue-black, blue and green specimens, many of which have deep overflows of other tones and interspersed mother-of-pearl.

History and origins

Opal is a stone of legendary beauty. In different parts of the world, in different lands and in different countries, there were special, peculiar to the ethnics of each individual people, but everywhere beautiful legends about disgrace.

• In ancient legends about Zeus, his victory over the titans is mentioned. Then Zeus experienced such great happiness that he wept, and the tears of the supreme god, falling to the ground, became rainbow opals.
• In Australia, the legend of the creator of the world is widely known. He descended into his creation with the intention of conveying to living beings all the knowledge available to him, and his power was so pure that every step was imprinted on the ground with iridescent stones.
• In India, the appearance of opals was attributed to the goddess of love. There was a legend according to which the goddess, running away from the men chasing her, fell to the ground and crumbled into beautiful stones.

A variety of beliefs led to the fact that the stone became a famous mystical symbol and became widespread not only in jewelry, but also in the magical sphere and the field of alchemy.

The very origin of the word "opal" is quite simple: it is the development of the word "stone" through the Sanskrit and ancient Greek languages.

Place of Birth

Stones are widely distributed throughout the world, they are often found as interlayers of rocks, however, there are few large miners of the rock.

Australia is the largest supplier of opals to the world market. Moreover, among all the varieties mined, white crystalline opals account for the most part, and somewhere around 5% of the industry belongs to black opals. An even smaller niche is occupied by expensive noble opals.

Ethiopia is a smaller supplier. In Honduras, Brazil, Mexico, Guatemala, the USA and Japan, a lot of precious opals are also mined, some of them are exported from Central Europe. are extracted in large quantities in Mexico, Turkey and Kazakhstan.

Physical properties

There are many varieties of opal, which differ in impurities in the structure of the material. Some of the rocks are saturated with iron and then opal is formed, some are formed by replacing the remains of wood and then a beautiful wood opal is formed, and some are formed from hot springs. There is also a species that is fully classified as precious stones. However, all these breeds have a number of common characteristics:

• All representatives of the breed are characterized by a pronounced mother-of-pearl luster, it can be dim and shine like glass.
• Stones have varying degrees of transparency: from slightly translucent specimens to fully transparent models.
• In white specimens, the surface noticeably shines with white, blue, yellow, pink tones, in black specimens - in different colors, in fiery specimens - in a brown or green tone.
• Opals do not have cleavage.
• According to the Mohs scale, opals are a fairly fragile material: their hardness is limited to 5.5-6.5.
• The pebbles have a rather fragile fracture.
• The structure of the material is sensitive to an alkaline environment and even dissolves in it.

The variety of breeds creates a wide variety of colors. The beauty of opals is their high-quality individuality: it is unlikely that you will find two pebbles alike.

Opal stone colors

Among the varieties of the rock there are stones with and without opalescence. The first ones shine with different colors and shimmer, the second ones do not. Opalescent stones are usually more expensive than simple counterparts and are valued much higher, although both types are quite beautiful.

This is the most expensive and one of the rarest types of precious noble opals. It has a dark base, which does not have to be exactly black. Often there are instances of dark blue, dark purple.

Transparent pebbles from puffy wine yellow to romantic hyacinth red. Usually they do not have mother-of-pearl overflows. The only place where opalescent fire opals are mined is Mexico.

This variety is almost opaque. The stones have a rich pearly white color with a pearly sheen. Often there are multi-colored inclusions in the structure of the material. The rock is widely extracted in the deposits of Russia, Armenia, Kazakhstan.

clear opal

Transparent options for opals are diverse: some types reveal their beauty only under water, some options have light color shades (for example, a blue tint or impurities of a noble brown tone).

This model has a rich yellow or amber color. Sometimes stones differ in rich overflows in tone.

Blue (Peruvian) opal

One-color Peruvian models are distinguished by amazing transparency. Shades of cast tone can vary from blue to dark blue.

This type of rock is usually opaque and somewhat similar to, only much brighter. The rich apple green color is popular with collectors.

Mother-of-pearl overflows can have different shades. Fiery red, blue, pink stripes are popular. Some stones may have circular streaks or any combination of reflective particles.

Amulets and talismans

Numerous legends and amazing beauty have made the stone a popular tool for creating talismans. From large breeds, representatives of the aristocracy ordered the manufacture of special figurines-amulets, which were designed to keep family peace, heartfelt virtue and save the owner from all troubles.

The magical properties of the stone

The stone has a strong, atmospheric energy, however, it can serve with dignity only to charismatic and talented people. Only in the hands of gifted people does he reveal his true deep properties, repelling everyone else and giving them only temporary superficial delight. The mystical properties of opals include:

Strengthens and directs creativity, protects it from external encroachments and influences of extraneous negative energies. People who are too pragmatic, prudent, living in a rigid rhythm, he will, on the contrary, disturb and excite too much. Calm, measured personalities will feel restless and perhaps even depressed.

• Black opals are considered stones of passion. They sharpen the mind, enhance analytical abilities, but at the same time awaken the vices of a person. The owner of black opal will walk a fine line between exposure to vices and use of the properties of the stone.
• White specimens are credited with a completely opposite character. This stone is assigned to healers, nurses. It enhances the owner's virtues, exacerbates his capacity for patience and compassion.
• Fire opals reveal all the characteristic features of red stones: masculinity, determination, self-sufficiency, courage in men, as well as wisdom, maturity and sensuality in women.
• Blue and blue stones bring undoubted good luck.

This is an ideal stone to help achieve goals, it will help to accumulate and redirect forces in the right direction for victory. According to ancient legends, he revealed to the owner the ability to see the future, so that he could take action to achieve results.

The healing properties of opals

In addition to their pronounced magical features, opals are also worn as a healing material. This stone:

• Improves stress tolerance. Normalizes mood, mental activity, balances emotional tone.
• Normalizes the nervous system. Clears the activity of the central nervous system from excessive nerve impulses and "cloggedness", puts in order the action of muscles, smoothes tics and chronic convulsions.
• Establishing the work of the central nervous system leads to an improvement in the condition of the heart muscle, the correct resumption of cardiac rhythm and the length of contractions.
• Establishes a relaxed and toned blood flow through the eyeballs, improves vision and general condition of the eyes.
• It improves the digestion of food, increases blood flow in the perigastric vessels and normalizes stomach contractions.

Opal and zodiac signs

Blue and white opals are recommended for fire signs. These stones are generally universal and suitable for almost any sign, however, in Aries, Leo and Sagittarius, they will smooth out natural flaws and enhance their natural determination, determination and courage. It is better for Aries to avoid opals of dark shades. Sagittarians will like blue specimens more.

Reasonable Libra, Gemini and Aquarius are ideal for both white and fiery stones. White specimens can make them too calm, but fiery colors will raise the necessary riot and add it to the character. Aquarians will also like dark blue stones.

Pisces and Scorpios can wear both red and blue-and-blue pebble models. These signs, by nature, are able to distinguish between danger and instantly recognize if the stone begins to cloud the mind. It is better for cancers to prefer opals of white tones.

Taurus will be impressed by blue opals, Virgos and Capricorns will like fiery specimens.

Black opals can only be worn by Cancers, Aquarius and Scorpios, the rest of the zodiac signs should not get carried away with them.

artificial opals

Artificial stones of poor quality can be made even in a home laboratory. In jewelry stores, of course, they provide much better fakes. Fakes often have a rougher structure, in which the play of colors occurs due to special “scales”.

How to distinguish a fake?

• Artificial stones shimmer in the light, like natural counterparts. However, their reflections and overflows are sharper, monochromatic, they are more “striking”, while the overflows of natural rocks are soft, with smooth transitions from one tone to another.
• Natural pebble in the sunlight casts a lot of small highlights on the hands, synthetic opal does not.
• You can often check for a fake using a simple method: touch the stone with the tip of your tongue, if the stone sticks, then it is a fake.
• In artificial specimens, cracks with air may be present, natural stones are usually solid.
• Natural stones have a clear zoning of tones.

Only a specialist equipped with the necessary equipment will help you to finally recognize the value of a mineral.

Noble opal and fire opal are a variety of opal with a bright interference color. Jewelry stone.

Usually noble opal and fire opal observed in the form of dense sinter formations of various shapes - crusts, reniform masses, oolites.

Opal Formula

SiO 2 nH 2 O

Noble opal (Precious opal, Noble opal). Characterized by a brilliant game of various colors - red, orange, green or blue - appearing in the form of sparkles, wavy areas or like flames, nets, etc. usually among the translucent or translucent main mass of milky white or other body ) coloring. Black opal is characterized by black or other very dark colors and play of colors. Harlequin opal has a variegated appearance with a mosaic pattern of rounded, angular or almost rectangular pieces of approximately equal size. In pin-fire-opal, the color is distributed in the form of closely spaced spots or tiny specks. Matrix opal or mother-of-opal is an opal cement containing closely spaced streaks or spots of opal; such stones are sometimes cut into cabochons or plates.

Fire Opal Concretion

fire opal

Fire opal (Fire opal). A variety of noble opal predominantly with a play of red or orange colors, usually on a rather transparent background with a pale yellowish, yellowish red, orange or brownish red body color. The name "fire opal" is incorrectly applied to transparent or highly translucent opals with an orange-red, brownish-red or red coloration that do not have a play of colors. These opals are found mainly in Mexico.

(Girasol, girasol opal).

A relatively transparent variety of noble opal with a fairly uniform bluish or reddish hue. variable coloration or with an undulating internal distribution of colors.

The name "jirazol", which comes from the Italian words girare - to rotate and sole - the sun, appeared relatively recently. At first, gemstones that sparkled in the sun, as well as fire opals, were called so, but now the name "jirazol" refers to opals with a uniform blurry play of bluish or reddish colors, usually on a slightly milky background.

opal Girasol

Physical properties of noble and fire opals
Optical

Mineral color. The purest opals are white and colorless transparent stones; usually there are milky-white or bluish-white opals. Depending on the presence of coloring impurities, they acquire a different color, usually pale shades: from yellow to yellowish-brown and brown, reddish-brown, green and blue, and also from gray to black. Sometimes opals are dyed red with finely scattered cinnabar or orange-yellow with orpiment. A brown or reddish-brown color is also common, due to iron oxides. In some opals (noble opals) there is a rich internal play of colors in reflected light - opalescence.

Firepal coloration is due to three reasons: pigmentation by finely dispersed foreign material, interference of light rays, in which the play of colors of precious opal is observed, and scattering of light, which leads to the appearance of a pale bluish tint in reflected light and a brownish tint in transmitted light. The cloudy milky white color of opals is due to the scattering of light. In any given specimen of opal, the coloration may be due to two or even all three of the above, but intense pigmentation with diffuse opaque foreign material can reduce the transmission of light so much that it becomes impossible to observe scattering or interference of light. In transparent and almost transparent types of precious opal, the play of colors is mainly due to the interference of light, although they sometimes show a faint color due to the admixture of a foreign coloring matter. For the hyaline variety of opal, neither coloring impurities of other substances, nor the phenomena of interference and light scattering are characteristic.

This discoloration is observed in reflected light and is not visible or very weakly manifested in transmitted light. Individual colors are often characterized by significant spectral purity and intensity. For some red colors, a spectral line with a width of about 20 A was measured. Color play does not occur under monochromatic illumination. Although these colors are caused by the interference of light rays, the nature of the structure responsible for this effect remains problematic. Possibly, this structure represents an open regular spatial network of cristobalite crystallites. However, some samples of noble opals give powder X-ray diffraction patterns of a crystalline substance, while others are X-ray amorphous. It was also pointed out that the play of colors may be due to the presence of thin interlayers with a refractive index that differs from the refractive index of adjacent interlayers, as well as systems of cracks or slits, possibly made by opal with a slightly different refractive index. The strong saturation of colors suggests that the structure that causes their occurrence has a periodicity of considerable magnitude. It is possible that the play of opal colors is associated with mechanical tensions that occur during the drying of the original gel and opal.

Dash color opal is white to faintly colored in colored varieties.

Glass luster. Wax finish, matte. Pearly iridescence.

Transparency

Opals vary in translucency from opaque to transparent, but mostly translucent and translucent stones are found.

Opal is isotropic, refractive indices and density vary with water content.

Mechanical

Hardness 5,5-6,5.

Opal is a rather brittle, sometimes noticeably brittle mineral.

Density

It fluctuates mainly depending on the water content, porosity and fracturing, usually from 1.99 to 2.25.

Fire Opal Deposits

Fire opal and noble opal are found in many places in the state of Querétaro, Mexico. Here, opals occur in the form of nodular and spotted accumulations in reddish-brown to pinkish rhyolites. Cavities in rhyolites can only be partially filled with opal, usually with characteristic banding in the form of alternating milky, hyaline, and other varieties. Sometimes in such cavities there are opal concretions that are not connected with the walls. Mexican, or Keretar, opals are very diverse in appearance. Body coloring of opals ( fire opal) varies from pale brownish-yellow, straw-yellow and reddish tones to orange, orange-red, brown, dark brownish-red and red. These opals, which are mostly quite transparent, often show a play of colors, usually reds and greens. Uniformly colored stones are sometimes used as a cutting material. The reddish and brownish shades of the body color of the fire opal are due to the admixture of iron oxides. Other Keretari jewelry opals are almost transparent stones with a bluish-white haze or with brilliant sparkles, or milky-white stones similar to Hungarian opals with brilliant color spots. Remarkable fire opals were also found near Zimapan, Hidalgo, and brought to Europe at the beginning of the 19th century; in 1807 Klaproth published an analysis of these opals. It is believed that Keretar opals first began to be processed at the end of the 19th century. In Aztec times, opal was used for mosaics and other purposes. Opals, including precious varieties, are widely distributed in Honduras, especially near Irandik. These stones, which form veins in trachytes, are usually represented by alternating layers of noble opal and common opal. The deposits of Honduras have been known since the beginning of the 19th century.

Numerous rich deposits of noble opal are known in Australia. In New South Wales, in the area of ​​the Lightning Ridge Mountains, there are deposits of wonderful black opals with a body color from gray to black with a bright play of red, green and dark blue or purple, or uniformly colored opals, iridescent in blue and dark purple tones. with separate green and red flashes.

In this area and at White Cliffs, common and noble opals occur as thin veins and veinlets, often extending along bedding planes and fissures in sandstones and conglomerates of the Upper Cretaceous Desert Sandstone Formation. Opal is mined here from sandstones lying near the surface. In White Cliffs, playful opals form pseudomorphs over large clusters of pineapple-shaped glauberite crystals. Opalized fossils are also found here, including belemnites, brachiopods, gastropods, and animal bones. Numerous occurrences of noble opal occurring in similar geological conditions are known in Queensland. Queensland opal usually has a milky or bluish-white body color with blue, green, and red sparkles. Noble opal is also found near the Anda flour, Coober Pedy and other areas of southern and western Australia. Some Australian opals, especially those from White Cliffs, exhibit a patterned distribution of color play. In a particularly valuable harlequin opal, the play of colors is distributed more or less regularly in variegated round, angular or roughly rectangular areas, while in point opals the play of colors appears in the smallest sparkles. Most Queensland and other Australian opals occur as rather thin veins or plates, which is why they are predominantly made not as cabochons, but as flat plates, often with other material on the reverse side. Australian opals generally have larger colored areas than Hungarian opals, as well as fairly large uniformly colored stones, though not to the same extent as most noble opals from Virgin Valley; in many cases two colors predominate, usually either red and green or green and blue.

Australian precious opal deposits

Almost all (more than 95%) noble opal in the world is obtained from deposits of ancient weathering crusts, and the vast majority of them are located on the territory. Geologically, this territory belongs to the so-called Great Artesian Basin, composed of Cretaceous and Tertiary deposits, along which the lateritic weathering crust is developed. Miocene age of the residual (eluvial) type, characteristic of peneplanated platforms. The Cretaceous and Paleogene rocks are crumpled into gentle folds with maximum angles of incidence of wings no more than 20°.

In the areas of deposits, weathering crust formations are subdivided by Australian geologists into three zones: lower, middle, and upper. The lower zone, up to 30 m thick, is composed of white and light gray kaolinite clays, gradually replacing the original sedimentary rocks of the Cretaceous and Paleogene. At its base (1-7 m) there are weakly weathered parent rocks - gray and brownish montmorillonite clays, as well as clayey and calcareous sandstones, sometimes containing layers of gypsum and lenses of quartz conglomerates. The middle zone is represented by variegated kaolinite material of red and yellow colors due to the appearance of iron oxides; its thickness ranges from 5 to 25-30 m. The upper zone of strong silicified rocks up to 15-20 m thick forms an armored shell on the tops of the hills; at the base, these are porcelain-like rocks, passing upwards into coarse-grained heterogeneous formations with a nodular spherulitic texture and signs of dissolution and redeposition of silica during climatic silicification. The total thickness of the weathering crust ranges from 6–15 to 50–60 m.

Accumulations of nodules, veinlets, and pseudomorphs of opal along shells, reptile bones, calcite, and gypsum are confined to the montmorillonite base of the weathering crust (LightningR id, Andamuka, and others), less often they are located even higher - among white and light gray kaolinite clays (White -Cliff c). The share of noble varieties in the total mass of mined opal is small (no more than 4-5%). They compose thin layers and veinlets 2-4 cm thick, form pseudomorphoses on bivalve shells, brachiopods, crinoids, plant remains, calcite and gypsum, and also fill cores and veinlets in siliceous concretions up to 20 cm in diameter. They are transparent and translucent, rarely black, more often dark gray, green, red, blue, blue and white formations with opalescence, causing a varied play of color in red, green and bluish tones; cracking ability is weakly expressed. Opals from Coober Pedy are most commonly red, orange and white, while those from Andamooka are green and blue; black opals are found at the Lightning Ridge deposit.

In association with noble opal are common opal, sericite, kaolinite, goethite, gibbsite, quartz, chalcedony, limonite, psilomelane, and alunite.

The geological appearance of the deposits is the same; the differences are only in the details, quality and extent of mineralization. Thus, the Lightning Ridge field (Wollangula) occupies a huge area
an area of ​​hundreds of square kilometers. It is overlain by alluvial Quaternary deposits, except for a protruding range of hills composed of Cretaceous sedimentary rocks.
(Winton Formation). In their composition from top to bottom, stand out;

• fine-grained white and cream-colored kaolinite rocks with quartz grains cemented with chalcedony and opal up to 5 m thick in surface outcrops (Kukoren mudstone);
• fine-grained white argillaceous rocks with rounded and grained white mudstones composed of quartz, chalcedony, altered feldspar and volcanic glass, 4 to 20 m thick or more (Wollangul sandstones);
• gray and brownish montmorillonite clays, sometimes colored red by iron oxides; form several layers with a thickness of 1.3 to 7 m (argillites F and inch).

Montmorillonite clays contain from two to five horizons with noble opal. The depth of occurrence of these horizons is from 12 to 30 m. Opal forms ellipsoidal nodules, rarely occurs in the form of layers, veinlets and pseudomorphs in flora and fauna. This is the only deposit where black opal is mined.

Most Australian geologists attribute the formation of these noble opal deposits to the ancient (Miocene) weathering of sandy-argillaceous rocks due to silica released during the decomposition of feldspars into kaolins. Opal was formed in the lower horizons of the crust when silica replaced all kinds of concretions, limestone and quartzite pebbles, calcareous shells, and other formations of the original rocks. As the crust deepened, the opal that was stable under these conditions persisted and accumulated in clayey zones. HeKofopbie geologists believe that the opal deposits in question are the result of modern climatic silicification, characteristic of desert regions with a hot dry climate. Under such conditions, rocks such as sandstones on well-drained plateaus turn into quartzites, and below, near the groundwater table, chalcedony or opal is formed.

Small deposits of precious opal have been found in many parts of the United States, such as near Opal Mountain in San Bernardino County, California; near Whelan in Leith County and in Lemhi and Owyhee Counties, Idaho; in Oregon. Opalized trees found in Yellowstone Park, Wyoming; in Socorro and Sierra counties, New Mexico; in Clover Creek, Lincoln County, Idaho; in the Leith Formation over a vast area in Klickitat, Yakima, and Benton Counties, Washington.