Opals are in a class by themselves. No other stone has such a rich folklore. (They have been considered both the luckiest and unluckiest gems to own). More than any other gemstone, each opal is distinctly individual. As a gem species, opal is so unique it has its own descriptive vocabulary. Opals are also the most delicate gems commonly worn and require special care.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Varieties||Common Opal, Agate Opal, Andean Opal, Cacholong Opal, Girasol, Honey Opal, Hyalite, Hydrophane, Jasper Opal, Milk Opal, Moss Opal, Pink Opal, Prase Opal, Semiopal, Wax Opal, Wood Opal, Fire Opal, Precious Opal, Black Opal, Boulder Opal, Crystal Opal, Harlequin Opal, Matrix Opal, Water Opal, White Opal|
|Crystallography||Amorphous. Recent work shows that opal is composed of an aggregate of tiny spherical particles, that is, a solidified gel; often forms concretions; botryoidal; reniform; stalactitic.|
|Refractive Index||1.44 - 1.47|
|Colors||Colorless, white, yellow, orange, and red (various shades), yellowish brown, greenish, blue, gray, black, violet.|
|Luster||Vitreous, waxy, pearly.|
|Polish Luster||Vitreous to resinous|
|Fracture Luster||Waxy, subvitreous|
|Fracture||Conchoidal to uneven|
|Hardness||5.5 - 6.5|
|Toughness||Fair to very poor|
|Specific Gravity||1.99 - 2.25|
|Density||1.99-2.25; orange—red variety ~2.00; black and white opal, 2.10; green opal, 2.12.|
|Cleavage||None. Fracture conchoidal. Brittle.|
|Luminescence||Green fluorescence in opal often due to included U minerals. Much opal fluoresces strong white in SW, LW, sometimes with persistent phosphorescence. (See table below.)|
|Special Care Instructions||Very heat senstive, clean with warm or room temperature soap and water. Avoid wearing gem where it will get rough treatment.|
|Absorption Spectrum||Most none. Green line at 660 nm, cutoff at 470 nm.|
|Phenomena||Play of Color. Asterism, chatoyancy, both rare.|
|Formula||SiO2 · nH2O. Water= 1-21% in opal, usually 6-10% in precious opal.|
|Optics||Isotropic; N: 1.44-1.47. Mexican opal as low as 1.37, usually 1.42-1.43.|
|Etymology||From the Latin name, opalus, for this stone, possibly derived from the Ancient Greek opallios for "color changing."|
|Occurrence||In sedimentary rocks or where low temperature solutions bearing silica can percolate through rocks.|
Opals have been mined and treasured by many cultures for thousands of years. Nevertheless, some scholars believe many ancient references to opal may actually have been to other gems, such as the iridescent iris agate.
Opal’s characteristic fire, or play of colors, was long thought to be the result of iridescence. However, with the advent of scanning electron microscopes, we now know it’s a result of diffraction. This phenomenon of flashing or moving colors due to diffraction is not related to the body color of the opal.
Opal is an amorphous form of silica, chemically similar to quartz, but containing 3% to 21% water within its mineral structure. Gem grade opals are usually 6% to 10% water content. Opal is a sedimentary stone. Under the proper conditions, water percolates through the earth, becoming rich in dissolved silicates. When it enters a cavity, the silicates are deposited as microscopic spheres, forming opals. If the spheres are uniform in size and shape and neatly stacked, they will diffract light. These stones are called precious opals. If the spheres are random in size, shape, and arrangement, the results are common opals.
The particular colors seen in an opal’s fire depend on the size of the spheres and the angle of viewing. For example, black opal gets its color from volcanic ash, but inclusions have nothing to do with the play of color. That is due entirely to the tiny silicate spheres. They must be smaller than 1,500 angstroms (Å) for blue and violet colors, but no larger than 3,500 Å to produce oranges and reds. To put that in perspective, 20,000 spheres are about the size of the period at the end of this sentence. (An angstrom is one ten-billionth of a meter).
Since opals grow by filling in cavities in the earth, regardless of their shape, sometimes they take the shape of pieces of wood, bone, and seashells buried in the ground. In effect, opal replaces these organic materials. These opals are called pseudomorphs, materials with shapes that are unrelated to their chemical content. Opalized wood is also called xylopal or zeasite.
Opal pieces that are too thin to use as a solid gemstones can be assembled into doublets and triplets. A doublet is a thin layer of precious opal glued to a black base. A triplet adds a transparent, quartz cap. Triplets make good ring stones because the hard quartz keeps the softer opal from scratching. Purists prefer the base material to be common opal. However many black materials are used, including old phonograph records.
The term “common opal” refers to opaque or glassy opals with a waxy luster. These stones come in a wide range of colors. They are seldom cut. Common opals are often fluorescent.
The term “precious opal” refers to opals of any color that display fire or a play of colors. They are also sometimes referred to as “noble opals.”
- Black opals have a black body color with fire, which is often spectacular against a dark background. Body colors can also be very dark blueish, greenish, or brownish. The name is also applied to black potch (inferior opal material) covered with a thin layer of crystal opal, which lets the black later show through.
- Gray or semi-black opal has light to dark body color with fire.
- White opal has a white body color with fire.
- Water opal has a transparent or colorless body that may have fire in it.
- Crystal opals are colorless and transparent to semi-transparent in transmitted light but with a rich play of colors in reflected light. Black crystal opals are transparent to semi-transparent with dark body color and play of color.
- Milk opal is translucent, milk white opal. These stones may also be yellowish or greenish in color.
- Fire opal is translucent to transparent with a yellow, orange, or red body color. These stones may or may not display a play of colors. The “fire” in their name refers to their body color, not to play of color. These stones are also called Mexican or sun opals.
- Boulder opals are thin seams of opal that form in ironstone. These opals come in many colors and show dazzling fire, backed by their brown ironstone matrix.
- Contraluz opals are very rare. They are usually found in Mexico but have also been found in Australia. They are transparent opals and show a play of color in both transmitted and reflected light.
- Hyalites are transparent to translucent, colorless or white to gray opals with glassy luster. They have little to no play of color. They are rarely faceted but generally have no gem significance. They are also called jelly opals.
- Moss opal is white to brownish and opaque. They contain dendritic inclusions that resemble moss.
- Hydrophane opals are light colored and opaque. When soaked in water or oil, they become transparent and show a display of color. They are sometimes called “magic stones.”
- Siliceous sinter or geyserite is glassy opal that forms around hot springs and geysers. These massive formations are not faceted for jewelry.
- Tripoli or diatomaceous earth are fine-grained, powdery masses of opal or the siliceous remains of microscopic marine animals called diatoms. This material is often used in polishing agents and fillers.
- Cachalong opal is porcelaniferous. (It resembles porcelain). It’s often blueish-white, translucent to opaque, and very porous. It will stick to your tongue. This stone is also called kalmuck agate opal.
- Jasper opal is reddish-brown and opaque. It resembles jasper.
- Prase opal is a green to yellowish green, translucent to opaque, common opal. It resembles chrysoprase. A golden-green variety is called chrysopal.
- Menilite is an opaque gray to brown opal with a concretionary structure.
- Tabasheer is an opaline (opal-like) silica found in the joints of some bamboo.
- Girasol opal is semi-transparent to translucent with a moving, billowy light effect. It resembles moonstone.
- Chrysocolla opals get their blue color from finely disseminated inclusions of chrysocolla, a copper phyllosilicate mineral.
- Liver opal is a term sometimes used for brown, common opal.
- Resin opal is a yellowish brown common opal with a waxy to resinous luster.
- Iron opal is a red to yellow common opal.
- Louisiana opal is composed of quartz, opal, and pyrite. It is found in the state of Louisiana.
- Oolitic opals have small black or brown spherical inclusions resembling fish roe and an over play of color.
- Wax opal is yellowish with a waxy luster.
- Star opals are extremely rare. The asterism they display is caused by imperfections in the arrangement of their silicate spheres, unlike the star effects of other gems such as sapphires, which are caused by inclusions.
Fluorescence in Opals
|White Cliffs, Australia||–||medium blue; phosphorescent|
|Park, Wyoming||dull white; phosphorescent||strong white; phosphorescent|
|Queretaro, Mexico||dull white; phosphorescent||bright blue; phosphorescent|
|Virgin Valley, Nevada||bright green||medium green, blue-white; phosphorescent|
|Quartzite, Arizona||pale yellow||bright pale yellow|
Opal may also fluoresce brownish. Black opal is generally inert. Fire opal luminesces greenish brown. Common opal often fluoresces green. Natural opals may have phosphorescence.
Descriptions of opal are as broad as the human imagination. The patterns seen in opals resemble stained glass windows, butterflies, birds, and other natural and human-made forms. Below are the most commonly used terms.
|Chinese writing resembles Asian script. Usually in gold and green, somewhat rare.|
|Fern leaf lacy patterns that resemble a fern leaf. Mostly in larger green gems, rarely red.|
|Flagstone large, broad patches of color, set close together. Much like harlequin, only the patterns are larger. Rare and highly valued.|
|Flame opal, bands or streaks of red fire that move across the surface.|
|Floral, a variety of repeated design and color, reminiscent of floral dress material.|
|Harlequin, or Mosaic, close set patches of broad, angular play of color. Highly desirable, adds considerable value.|
|Pallet has a variety of colors, resembling an artist’s paint pallet.|
|Peacock opal, predominantly green and blue play of color, with some resemblance to a peacock tail.|
|Pinfire or Pinpoint, very small areas of fire set close together. Common and one of the less valuable patterns.|
|Rolling Fire a rare phenomena, where the fire moves across the surface rather than flashing on and off.|
|Ribbon slightly curved, parallel bands of color. Usually parallel and on black or dark base. Rare.|
|Straw resembles flattened straws, crisscrossing each other. Rare.|
|Windmill a pattern of fire that radiates around a central point. Very rare.|
- Chaff, thin, linear bands of color.
- Flash opal, play of color that quickly flashes off and on as the stone is moved.
Terms for Color and Color Distribution in Opal
- Onyx opal and agate opal: alternating layers of precious and common opal. In cat’s eye opal, the color play is concentrated in the form of an eye or band.
- Matrix opal: consists of specks of precious opal in a rock matrix, usually sandstone; this type of opal is often dyed black to enhance the color play. Ironstone opal is in a brown, hard, compact type of sandstone. Matrix opal may also be layers or stringers of opal in a rock matrix. Also called mass opal.
- Also gold opal (gold fire), blue opal (blueish fire), lechosos opal (green colors).
Synthetic opals may show a strong display of color, usually in a mosaic pattern. Under high magnification with top or backlighting, a cellular, scale-like, snake skin, or chicken wire structure can be found in the pattern. Under high magnification with transmitted light, synthetics may show a dendritic structure.
Synthetic opals don’t phosphoresce. They may also stick to the tongue. Synthetic white opal can show columnar structures from the side.
Plastic imitations or simulants are soft and can be probed with a sharp needle. They don’t phosphoresce.
- Impregnated with oil, wax, or plastic: improves play of color, disguises crazing. Common. Stability poor for oil and wax. Can be detected with a hot point test. Plastic usually requires major lab equipment.
- Impregnated with black plastic: gives appearance of black opal, disguises crazing. Stability excellent. Somewhat common. Can be detected because the piece’s SG is too low, concentration of color in cracks. May require major lab equipment.
- Impregnated with smoke: darkens body color. Common. Stability, fair to poor. Can be detected through magnification, low SG, (may float in water until it absorbs enough to sink), loses play of color when wet, returns when dry. Unusually low RI, 1.38 to 1.39.
- Treatment with aniline dye, silver nitrate, or sugar: darkens body color. Common. Stability good. Magnification shows black concentrations.
- Hydrophane opals will absorb water and chemicals, which can have a negative affect on their appearance. They must be kept dry and away from all sources of contamination.
- Honduras: deposits known since before 1843, perhaps much older than that. Occurs as veins in dark reddish to black trachyte rock. White opal contrasts strongly with the dark-colored matrix. Pieces not large, seldom very spectacular.
- Czech Republic and Slovak Republic: source of opal known in Roman times, near the village of Czerwenitza (formerly in Hungary). Opal occurs as seams in grayish brown andesite rock. The opal is a mosaic of strong colors and is very attractive, against a milky-white background color. Much of this is harlequin opal.
- Ethiopia: an ancient source of opals, recent discoveries of precious opal in 1994 and 2008. The opal found near the town of Wegel Tena displays a remarkable play of color.
- Indonesia: very little known material, as thin seams in dark rock. Much of it is water opal and resembles material from Mexico. The white opal resembles poor-grade Australian. Some black opal is produced that is very unusual and consists of reddish flecks of color swimming in a translucent but very dark brown body. Most gems are very small (less than 10 carats) from this locality, and production is very small.
- Mexico: Mexican opal occurs in siliceous volcanic lavas, in cavities, and in many localities. Yellow and red fire opal comes from a trachyte porphyry at Zimapan in Hidalgo. Hyalite and precious opal that is completely transparent, colorless, and rich in fire occurs at San Luis, Potosi, Chihuahua. Queretaro is a well-known opal-producing locality. Fine Mexican opal is very rare in large sizes (over 50 carats) but is among the most beautiful.
- Virgin Valley, Nevada: opal occurs in Humboldt County as cracks and seams in opalized Wood. This was discovered about 1900. The opal is magnificent, but is very hydrous and has a strong tendency to crack due to loss of water when exposed to the air. This behavior is known as crazing, or, when on the surface, checking. Whole skeletons of extinct animals have been replaced by fine precious opal at this locality. Similar opal is found in Idaho.
- Brazil: opal occurs in sandstone in Piaui Slate, northern Brazil, and also near Manaus, northern Brazil. The material is white and fiery and sometimes resembles good-quality Australian white opal. It is perhaps the most durable opal, low in water and not heat sensitive I have seen a cut gem held for a half a minute over a candle flame with no adverse effects. The material seems to be abundant and much of it is shipped to Hong Kong where it is cut and sold, often as Australian opal.
- Poland: green prase opal, colored by nickel.
- Tanzania: nickeliferous opal resembling chrysoprase occurs in Tanzania, associated with brown limonite. The R.I. (1.452) is lower than that of chrysoprase (1.535), as is the gravity (2.125 versus 2.620). Stone sizes tend to be small.
- Australia: the first discoveries were probably about 1850. but major finds were made in Queensland about 1872. Australian opal is in various types: Boulder opal: shells of coarse, hardened, sandy clay with layers of opal in between. Yowah nuts: walnut-sized concretions, in a regular layer, like a conglomerate. The opal is the central kernel and never reaches the outer edge. Seam opal: thin to thick seams of white or black opal in sandstone matrix. Also known as sandstone opal. Large stones are very rare in this material.
- Major finds of Australian opal are best known from specific localities: Lightning Ridge: black opal in nodules, world‘s finest of this material; first mined commercially about 1905. The Grawin opal field, about 25 miles SW of Lightning Ridge, produces light-colored seam opals. Coober Pedy, South Australia: discovered about 1915; only white opals found here, in sandstone and claystone matrix, but some very fine. Andamooka, South Australia: opals found here about 1930; very distinctive opal, white and also brownish in color; may be artificially blackened-to enhance the appearance of the fire in the matrix. White Cliffs area: started about 1889, but the opal is usually small, with veinlets of precious opal within common opal.
- Gabanintha (Murchison Goldfield): bright green opal, colored by copper, is found in quartz. Mintabie: mined since 1931, about 350 km northwest of Coober Pedy. This area has now been extensively prospected.
Australia is the best known opal-producing area in the world, but the deposits have been worked so intensely that they are becoming depleted. Fewer miners are now working the opal fields than 10 years ago, and new discoveries are rare. This factor, plus worldwide demand, is putting tremendous pressure on opal prices.
Here are a few large and fine opals that have been given individual names, like diamonds.
- Olympic Australis: Coober Pedy; uncut was 127 oz.
- Noolinga Nera: Andamooka; 86 oz. Rough cut, cut 205 carat oval.
- Roebling Opal: Nevada (Rainbow Ridge); 2610 (in the Smithsonian Institution).
- Light of the World: Lightning Ridge, Australia; 252, partly cut
- Red Admiral or Butterfly: Lightning Ridge, Australia; 40-50 in rough. Many regard this as the world’s most beautiful opal.
- Pride of Australia: Lightning Ridge, Australia; 226, partly cut.
- Pandora: Lightning Ridge, Australia; 711, cut
- Smithsonian Institution (Washington, DC): Australian gems; 345, 155, 83 (all white); black opals of 58.8, 54.3, 44. Also 355 (black, Nevada); 143.2 (orange, precious, Mexico); 55.9 (colorless, precious, Mexico); 39 (pale yellow-orange, precious, Brazil).
Opals are delicate but well worth the extra care. They are very sensitive to changes in temperature. They also have a crazing tendency. They “craze” or develop cracks as they dehydrate. Opals that have been kept in water will need to be dried carefully before they can be cut.
Sometimes opals in rings can become chalk white and lifeless. This may be due to a network of scratches on the opal surface that destroys the polish and dulls the color play. A simple re-polishing can usually correct this. Be advised that opals can be shattered or damaged beyond repair much more easily than other popular gemstones. They are have a hardness of only 5.5. Opals are not recommended for ring stones, unless the stone is placed in a protective setting or a triplet.
Consult our care guide for opals for cleaning, setting, and storage recommendations.
by Dr. Joel Arem; Donald Clark, CSM IMG