Opal

Hardness: 5.5-6.5 on Mohs scale; Color: White, colorless, blue, green, yellow, orange, red, black (most commonly milky white as seen in the image); Luster: Vitreous to waxy/resinous; Crystal structure: Amorphous (no defined crystal structure); Cleavage: None; Specific gravity: 1.98-2.25

Opal forms from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone, and carries this silica-rich solution into cracks and voids, caused by natural faults or decomposing fossils. As the water evaporates, it leaves behind a silica deposit. This cycle repeats over very long periods of time, and eventually, opal is formed. Most precious opals are found in sedimentary rocks, particularly sandstones and limestones, or in volcanic rocks. The formation process is typically Cenozoic, though some older occurrences exist.

Opal is primarily used as a gemstone in jewelry due to its unique play-of-color and aesthetic appeal. Common opal, which lacks the play-of-color, can be used for cabochons or carving. It has limited industrial uses, though synthetic opals are sometimes used in optics.

Australia produces about 95% of the world's precious opal, with significant deposits in Coober Pedy, Lightning Ridge, and Mintabie. The 'play-of-color' in precious opal is caused by the diffraction of light by a regular array of silica spheres within the stone. 'Common opal' or 'potch' does not exhibit this play-of-color and is usually opaque or translucent with a milky sheen, similar to the specimen pictured.

In the field, common opal can be identified by its often milky white, translucent to opaque appearance, waxy luster, and relatively low hardness compared to quartz. It usually feels lighter than other minerals of similar size due to its lower specific gravity. It is often found in nodules or seams within sedimentary or volcanic rocks. For collectors, distinguishing common opal from precious opal is crucial; the play-of-color is the key indicator of precious opal. The specimen shown appears to be a tumbled or polished piece of common opal, likely exhibiting a milky or slightly yellowish translucent quality. Common opal is found worldwide, with notable deposits in the US (Idaho, Nevada), Mexico, Ethiopia, and other volcanic and sedimentary regions.