Precious Opal

Precious Opal is the gem-quality opal defined by its play-of-color: flashes of spectral hues that shift as the stone moves. It is distinguished from common opal (potch), which lacks this effect.

Like all opal it is amorphous hydrated silica, holding several percent water. The body can be black, white, gray, blue, or transparent, with the play-of-color overlaid on top.

Precious opal is one of the most distinctive of all gemstones, and fine examples, especially black opal with vivid color, rank among the most valuable gems by weight.

Precious Opal forms when silica-rich water percolates through rock and slowly deposits silica gel in cavities, seams, and around fossils. The play-of-color appears only when the silica microspheres are nearly uniform in size and arranged in an orderly, repeating lattice that diffracts light.

This precise ordering is uncommon, which is why precious opal is far rarer than common opal. Sphere size determines the colors produced, from blue-violet through to the scarce reds.

Australia is the dominant source, with famous fields at Lightning Ridge, Coober Pedy, and Andamooka; Ethiopia's Welo region is another major modern producer.

The defining test is play-of-color: rotate the stone and look for flashes of spectral color that move and change, distinct from the metallic sheen of labradorite or moonstone's floating glow.

Hardness is 5.5-6.5, luster vitreous to waxy, and streak white. Body color varies widely.

Key look-alikes are imitations and assembled stones (doublets and triplets). Inspect the edge for glued layers, and note that natural precious opal shows three-dimensional, irregular color play rather than the regular, lizard-skin pattern of some synthetics.

Precious Opal is among the most celebrated jewelry gems, cut as cabochons for rings, pendants, and brooches. Value depends on body tone (black being most prized), brightness, color range, and pattern.

Its softness and water content demand care: avoid impacts, heat, chemicals, and drying conditions that cause crazing. Solid stones are more durable than thin doublets or triplets.

Opal is the traditional birthstone for October and is wrapped in folklore about hope and creativity. Its true distinction, however, is the singular optical phenomenon of play-of-color.