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Search Location   All Locations California Colombo Ohio Category   All Categories Alexandrite Alexandrite Catseye Amethyst Ametrine Andalucite Apatite Apatite Cat's Eye Aquamarine Aquamarine Cat's Eye Bargain Lots Beryl Chrysoberyl Chrysoberyl Catseye Chrysoberyl Stars Citrine Cobalt Spinel Crystals Diopside Diopside Cat's Eye Diopside Star Enstatite Feldspar Fluorite Garnet Garnet Color Change Garnet Grossular Garnet Hessonite Garnet Rhodolite Garnet Spessartine Garnet Star Garnet Tsavorite Investment Gems Iolite Konerupine Kunzite Kyanite Lime Citrine Moonstone Mystic Topaz Opal Padparasha Pairs Peridot Quartz Quartz Catseye Quartz Stars Rare Gems & Minerals Ruby Ruby Star Rutile Sapphire Blue Sapphire Cats Eye Sapphire Green Sapphire Lavender Sapphire Other Sapphire Pink Sapphire Purple Sapphire White Sapphire Yellow Scapolite Scheelite Silimanite Silimanite Catseye Sinhalite Special Cut Gems Sphene Spinel Spinel Star StarSapphire Taaffeite Topaz Topaz Catseye Tourmaline Tourmaline Bi-Color Tourmaline Chrome Tourmaline Col/Chg Zircon       Alexandrite Enstatite Opal Scapolite Alexandrite Catseye Feldspar Padparasha Silimanite Catseye Amethyst Fluorite Peridot Sinhalite Ametrine Garnet Quartz Sphene Andalucite Garnet Color Change Quartz Catseye Spinel Apatite Garnet Grossular Quartz Stars Spinel Star Apatite Cat's Eye Garnet Hessonite Ruby Star Sapphire Aquamarine Garnet Rhodolite Ruby Star Taaffeite Aquamarine Cat's Eye Garnet Spessartine Rutile Topaz Beryl Garnet Star Sapphire Blue Topaz Catseye Chrysoberyl Garnet Tsavorite Sapphire Cats Eye Tourmaline Chrysoberyl Catseye Iolite Sapphire Green Tourmaline Bi-Color Chrysoberyl Stars Konerupine Sapphire Lavender Tourmaline Col/Chg Citrine Kunzite Sapphire Other Zircon Cobalt Spinel Kyanite Sapphire Pink     Diopside Lime Citrine Sapphire Purple     Diopside Cat's Eye Moonstone Sapphire White     Diopside Star Mystic Topaz Sapphire Yellow       Opal is a poorly crystalline or amorphous hydrous silica that is compact and vitreous and most commonly translucent white to colourless. Precious opal reflects light with a play of brilliant colours across the visible spectrum, red being the most valued. Opal forms by precipitation from silica-bearing solutions near the Earth's surface. Electron microscopy has shown that many opals are composed of spheres of tens to a few thousand angstroms in size that are arranged in either hexagonal or cubic close packing. The spheres are composed of hydrous silica that may be either almost cristobalite-like, tridymite-like, mixtures of both, or random and nondiffracting. The specific gravity and refractive index are lower than those of pure silica minerals. The play of colours in precious opal arises from the diffraction of light from submicroscopic layers of regularly oriented silica spheres. When heated, opal may lose as much as 20 percent of its weight of water, fracture, and then crystallize to one of the silica minerals described above. Gemological Information Color : White, green, blue, black, red, orange, violet, pink, grey, yellow. Refractive Index: 1.44-1.46 Chemical Composition: SIO2nH2O Hardness: 5.5 - 6.0 Density: 2.65 - 3.00 Crystal Group: Amorphous Ocurrence: Czechoslovakia, Mexico, Honduras, Australia, Brazil, Tanzania. Opal usually contains 4 to 9 percent water, but lower and much higher values have been observed. The contents of alumina, ferric oxide, and alkalis are variable but may amount to several percent in light-coloured opals and more if pigmenting minerals are also present. Precious opal has been synthesized. Opaline silica is a friable hydrous silica found near hot springs and geysers.