Opal October’s Birthstone
Formula: SiO2 · nH2O. Water= 1-21% in opal, usually 6-10% in precious 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.
Colors: Colorless, white, yellow, orange, and red (various shades), yellowish brown, greenish, blue, gray, black, violet.
Luster: Vitreous, waxy, pearly.
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.
Optics: Isotropic; N: 1.44-1.47.
Mexican opal as low as 1.37, usually 1.42-1.43.
Dispersion: Very low.
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.
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.)
Refractive Index 1.44 – 1.47
Specific Gravity 1.99 – 2.25
Heat Sensitive Very
Special Care Instructions Very heat senstive, clean with warm or room temperature soap and water. Avoid wearing gem where it will get rough treatment.
Enhancement Impregnated with oil, wax, or plastic. Occasional. Smoked, to create black opal. Occassional. Treated with dye or chemicals to make light opal black. Occasional.
*Wearability is graded as Excellent, Very Good, Good, Poor, and Forget It! For more details see the article on “Hardness and Wearability.”
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.
Opals are delicate, but well worth the care. Their most significant weakness has to do with the water content. If an opal is allowed to dry, it will crack and craze. In most cases, they do not need any special care while stored. However, if you live in a very dry climate, or keep them in a dehumidified room, some precautions are necessary. Keeping them in a tight plastic bag, with a damp piece of cotton or fabric will prevent dehydration.
Storing an opal in oil or glycerin is not recommended. It is unlikely to damage the opal, but it is unnecessary and requires tedious cleaning.
Because of their water content, opals are also highly sensitive to sudden changes in temperature. I know of a woman in Pennsylvania who wore a brooch on the outside of her coat. As she passed from the warmth of her house to the winter cold, there was an audible “crack” as her opal self-destructed.
Opals do not mind being hot or cold, it is the rate of change that damages them. You need to avoid situation like the one above, going from a warm house to the winter’s cold. Simply wearing an opal under clothing will protect them. Also, do not store opals near a heat source, an open window, etc., where they can be exposed to sudden temperature changes.
(Click Images below to see some more examples)
Being somewhat soft, they scratch easily. Realize that a large component of dust is quartz at 7 in hardness. At 5.5 to 6 in hardness, simply wiping the dust off an opal will gradually reduce its polish. The solution is to clean your opals using a soft cloth or brush, a mild detergent, and room temperature water. Then rinse the jewels to remove any residue. Clean doublets and triplets with the same method, but do not soak them. Soaking can dissolve the glue holding the layers together.
Prevention is the best solution to scratching and chipping. Opals are best suited for earrings, brooches, and pendants. These jewels receive little contact with harder objects, compared to what a finger ring experiences. If you do get an opal ring, choose a setting that protects it from coming in contact with other objects.
Understand that, if you wear an opal ring on a regular basis, it will require occasional repolishing. Reserving your ring for special occasions will greatly reduce the risk of damage.
Make sure you remove your ring before physical activities like gardening and sports. Also, do not immerse the gem in liquid chemical solutions, like dishwater. Opals are porous and absorb liquids.
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.
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).
Occurrence: 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.
Many thanks to Dr. Joel Arem, for allowing IGS to use some of the Opal images from his Color Encyclopedia of Gemstones.