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That is due entirely to the tiny silicate spheres. For example, black opal gets its color from volcanic ash, but inclusions have nothing to do with the play of color. The particular colors seen in an opal’s fire depend on the size of the spheres and the angle of viewing. This phenomenon of flashing or moving colors due to diffraction isn’t related to the body color of the opal. However, with the advent of scanning electron microscopes, we now know it’s a result of diffraction. Opal’s characteristic fire, or play of colors, was long thought to be the result of iridescence. Precious opals, also known as “noble opals,” display fire or play of colors. Seldom cut, these stones come in a wide range of colors. If the spheres are random in size, shape, and arrangement, the results are common opals.Ĭommon opals can have an opaque or glassy appearance with a waxy luster.
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If the spheres are uniform in size and shape and neatly stacked, they will diffract light. When water enters a cavity, it deposits the silicates as microscopic spheres, forming opals. Under the proper conditions, water percolates through the earth, becoming rich in dissolved silicates. Gem grade opals usually have 6% to 10% water content. Opal is an amorphous form of silica, chemically similar to quartz, but containing 3% to 21% water within its mineral structure. What’s the Difference Between “Common Opals” and “Precious Opals”? In sedimentary rocks or where low temperature solutions bearing silica can percolate through rocks.īlack opal, Australia (stone in bracelet ~ 20 carats). Mexican opal as low as 1.37, usually 1.42-1.43.įrom the Latin name, opalus, for this stone, possibly derived from the Ancient Greek opallios for "color changing." Water= 1-21% in opal, usually 6-10% in precious opal. In rare cases, can display chatoyancy or asterism. Avoid wearing gem where it will get rough treatment. Very heat senstive, clean with warm or room temperature soap and water. (See table below.)įluorescent, Phosphorescent, UV-Long, UV-Shortĭyeing, Infusion/Impregnation, Surface Coating
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Much opal fluoresces strong white in SW, LW, sometimes with persistent phosphorescence. Green fluorescence in opal often due to included U minerals. (Orange-red variety, ~2.00 black and white opal, 2.10 green opal, 2.12). 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.Ĭolorless, white, yellow, orange, and red (various shades), yellowish brown, greenish, blue, gray, black, violet.ġ.99-2.25. © Rob Lavinsky, Used with permission.Ĭommon 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 OpalĪmorphous. 78 opal cabochons from Welo, Ethiopia (52 ctw) in 18k gold bracelet.