Opal is hydrated silicon dioxide – submicroscopic silica spheres, bonded together with H2O and then more silica, sometimes containing the minerals cristobalite and tridymite. It is amorphous, meaning it does not have a crystalline structure and does not have a definite chemical composition and is therefore considered to be a “mineraloid” rather than a “mineral.” Over time, this gelatinous mixture settled into cracks, into veins and between the layers of sedimentary rock under the ground. Then the solidified material, Opal, formed as much of the water evaporated. In some rare circumstances, Opal formed in masses exhibiting botryoidal growth, in stalagmitic form, or replaced some materials in fossils. Even after solidifying, Opal maintains a water content ranging from 3% to 21% by weight, usually between 6% and 10%.
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Measurements are approximately: 2 inches wide,5 inches long, and .50 of an inch thick