Opal ValueValuing precious opal is a highly refined art. For full details, see Appraising Opals – Part 1, Nomenclature and Appraising Opals – Part 2, Applying Value
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|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.|
Isotropic; N: 1.44-1.47. Mexican opal as low as 1.37, usually 1.42-1.43.
Fluorescence in Opal
|White Cliffs, Australia||
medium blue; phosphorescent
dull white; phosphorescent
strong white; phosphorescent
dull white; phosphorescent
bright blue; phosphorescent
|Virgin Valley, Nevada||
medium green, blue-white;phosphorescent
bright pale yellow
Opal may also fluoresce brownish. Black opal is generally inert. Fire opal luminesces greenish brown. Common opal often fluoresces green.
Comments by Donald Clark, CSM IMG
Opals stand in a class by themselves. More than any other gem, each opal is distinctly an individual. No other stone has as rich and varied folklore. They are both one of the luckiest and unluckiest gems a person can own. They are so unique, they have their own descriptive vocabulary. Opals are also the most delicate gems commonly worn. They require special care to insure their health and longevity.
The name evolved from the Greek “Opallus” which means to see a change in color. Later, the Latin word “opalus” came to mean precious stone.
There is some doubt that ancient authors were referring to the same stone we call opal today. Some scholars believe that many references are actually to iridescent gems, like iris agate.
Opal’s fire was long thought to be the result of iridescence. However, with the advent of scanning electron microscopes, we now know that it is a result of diffraction.
Opal is an amorphous form of silica, (SiO2.nH2O) chemically similar to quartz, (SiO2) but containing 3% to 21% water within the mineral structure. Gem grade opals are usually 6% to 10% water content.
Opal is a sedimentary stone. Under proper conditions, water percolates through the earth, becoming rich in dissolved silicates. When it enters a cavity, the silicates are deposited as tiny spheres. If they are uniform in size and shape, they will diffract light. If they are random in shape and arrangement, we have common opal.
Volcanic ash gives black opal its color, but inclusions have nothing to do with the play of color. That is due entirely to the tiny spheres. They must be smaller than 1500 angstroms for blue and violet colors, but no larger than 3500 angstroms 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.
Opal grows by filling in cavities, regardless of their shape. Hence, we have many pseudomorphs, materials with shapes that are unrelated to the chemical content. The most common are opalized wood and seashells.
(Click Images below to see some more examples)
Opals are delicate but well worth the extra care. Consult our care guide for pearls and opals for cleaning, setting, and storage recommendations.
- Siliceous sinter; geyserite: massive, glassy opal that forms around hot springs and geysers; no gem significance.
- Diatomaceous earth; tripoli: fine-grained, powdery masses of opal or the siliceous remains of microscopic marine animals called diatoms. Often used as polishing agents, fillers.
- Pseudomorphs: opal may, in percolating through the ground, replace (on a microscopic or even cellular basis) wood, bone, and shells.
- Hyalite: transparent, colorless, or white to gray opal, glassy, occasionally faceted but generally no gem significance.
- Common opal: opaque or glassy opal, in a wide range of colors, sometimes with a waxy luster; often fluorescent; seldom cut into gems.
- Water opal: transparent, colorless opal that may have fire in it.
Note: The term fire refers to the magnificent play of color displayed by opal, which is due to light diffraction from neatly stacked layers of the microscopic spheres of which opal is composed. Common opal is a jumble of spheres of random sizes, but in precious opal the spheres are the same size, and they are layered in neat rows. The particular color seen depends on the size of the spheres and the angle of viewing.
- Fire opal: transparent to translucent red or orange opal, which may or may not have fire in it! The term fire opal refers to a body color, not to play of color.
- Precious opal: opal of any color with fire or play of colors displayed.
- White opal: white body-color opal, usually with play of color.
- Gray opal: light to dark body color, with play of color superimposed.
- Black opal: black body color with fire, often spectacular against dark background. Body color also very dark bluish, greenish, or brownish.
- Semiblack opal: another way of describing gray opal.
- Milk opal: milk white, translucent, also yellowish or greenish in color.
- Crystal opal: water opal or milk opal , generally rich in fire; transparent to translucent in transmitted light; colors seen by reflected light.
- Contra-luz opal: very rare type, usually from Mexico, with color play in both transmitted and reflected light.
- Hydrophane: light colored, opaque, becomes iridescent and transparent when soaked in water.
- Jasper opal: reddish-brown opal, opaque, resembles jasper.
- Cachalong: porcelaniferous, often bluish-white, very porous—adheres to the tongue.
- Prase opal: translucent or opaque green opal; a common opal resembling prase.
- Moss opal: white to brownish opaque opal that contains dendritic inclusions.
- Menilite: opaque gray to brown opal with a concretionary structure.
- Tabasheer: opaline silica occurring in the joints of bamboo.
- Girasol: opal that is almost transparent and has a billowy light effect within it, resembling moonstone.
- Chrysocolla in opal: blue material, with finely disseminated chrysocolla that gives the color.
- Liver opal: term sometimes used for brown common opal.
- Resin opal: yellowish brown common opal with a waxy luster.
- Play of color: the phenomena of flashing or moving colors due to diffraction and not related to the body color.
- Noble opal, or Precious opal: Opal with play of color.
- Crazing: cracks that develop as an opal dehydrates.
Doublets and Triplets make use of opal that is too thin to use as a solid gemstone. A doublet is a thin layer of precious opal glued to a black base. A triplet adds a transparent, quartz cap.
Purists prefer the base material to be common opal. However many black materials are used, including old phonograph records.
- Black opal, precious opal with black body color. Also used for black potch covered with thin layer of crystal opal that lets the black under layer show through.
- Semiblack, or gray opal, precious opal with dark body color.
- White opal, precious opal with white body color.
- Crystal opal, transparent to semitransparent, colorless body with play of color.
- Black crystal opal, transparent to semitransparent, with dark body color and play of color.
- Fire opal, Translucent to transparent, with yellow, orange, or red body color. May or may not have play of color. Also called Mexican opal or Sun opal.
- Boulder opal, thin seam of opal naturally occurring on ironstone matrix.
- Cachalong opal, translucent to opaque, bluish white opal that resembles porcelain. Very porous, will stick to tongue. Also called Kalmuck agate opal.
- Chrysocolla opal, blue Chrysocolla, finely distributed in common opal.
- Contra-luz opal, transparent opal showing play of color in both reflected and transmitted light.
- Girasol opal, semitransparent to translucent opal with moving, billowy light resembling moonstone.
- Gold opal, fire opal with gold body color or precious opal with only gold fire.
- Hyalite, transparent to translucent, white common opal. Also called Water stone or Water opal.
- Hydrophane, common opal that shows play of color when soaked in water or oil. Also called Magic stone.
- Iron opal, red to yellow, common opal.
- Jasper opal, reddish brown, common opal.
- Jelly or Water opal, colorless, transparent to semitransparent with little or no play of color.
- Lechoses opal, has only green play of color.
- Liver opal, brown, common opal.
- Louisiana opal, composed of quartz, opal and pyrite. Found in Louisiana, USA.
- Matrix opal, thin seams or spots of precious opal in matrix, (usually ironstone.) Also called Mass opal.
- Menilite, opaque gray to brown opal with concretion structure.
- Milk opal, white common opal.
- Moss opal, common opal with inclusions resembling moss.
- Onyx opal or Agate opal banded layers of precious and common opal.
- Oolitic opal, a stone with small black or brown spherical inclusions resembling fish roe, over play of color.
- Opalized wood, bone, or shell, pseudomorphs where organic material is replaced by opal. Opalized wood also called xylopal.
- Prase opal, green to yellowish green common opal resembling chrysoprase. Also called Chrysopal.
- Resin opal, yellowish brown opal with resinous luster.
- Rolling Flash Fire play of color travels across surface as stone is moved. Very rare.
- Siliceous sinter, glassy opal that forms around hot springs and geysers. Also called Geyserite.
- Tabasheer, opal like silica found in the joints of some bamboo.
- Tripoli or Diatomaceous earth, fine grained opal and siliceous remains of diatoms. Use as a polish or filler.
- Wax opal, yellowish opal with waxy luster.
- Zeasite, opalized wood.
Note: the descriptions of opal are as broad as the human imagination. Patterns include stained glass windows, butterflies, birds, and other natural and man-made objects. Below are the most common 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 catseye 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.
- Flame opal: sweeping reddish streaks and bands move across the gem, resembling flickering flames.
- Flash opal: as the gem is moved back and forth,
- flashes of color appear and disappear at various spots.
- Harlequin opal: the color display is in the form of angular or quiltlike patches, all in contact with each other, like a mosaic.
- Pinfire opal: the color is in the form of tiny dots or speckles, set close together.
- Peacock opal: many colors appear in the same gem, resembling the display of the tail of the male peacock.
- Also gold opal (gold fire), blue opal (bluish fire), lechosos opal (green colors).
Impregnated with oil, wax, or plastic. Improves play of color, disguises crazing. Common. Stability poor for oil and wax.
Detect oil and wax with hot point. Plastic usually requires major lab equipment.
Impregnated with black plastic. Give appearance of black opal, disguises crazing. Stability excellent. Somewhat common.
Detection, SG too low, concentration of color in cracks. May require major lab equipment.
Impregnated with smoke. Darkens body color. Common. Stability, fair to poor.
Detection, magnification, low SG, (may float in water until it absorbs enough to sink,) looses 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.
Detection, magnification shows black concentrations.
Hydrophane opals, (common to Ethopia,) will absorb water and chemicals which have a negative affect on their appearance. They must be kept dry and away from all sources of contamination.
Play of color. May have phosphorescence.
Strong play of color, usually in mosaic pattern. Inside mosaic there is a cellular, scale-like, snake skin, or chicken wire structure. (High magnification, top or backlighting.) May show dendritic structure, (high magnification, transmitted light.)
Does not phosphoresce. Sticks to tongue. White opal shows columnar structure from side.
Soft, can probe with sharp needle. Does not phosphoresce.
Typical glass bubbles and swirl marks. Does not phosphoresce. SG and RI usually higher.
In sedimentary rocks or where low temperature solutions bearing silica can percolate through rocks.
- 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.
- Czechoslovakia: 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.
- 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. 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. Many 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.
Source/Attribution: Dr. Joel Arem, Donald Clark, CSM IMG