Common Opal

Common opal, often called potch, is opal that lacks the rainbow play-of-color seen in precious opal. It is valued instead for its solid body color, which can be white, gray, yellow, green, blue, pink, brown, or black, and for varieties like moss opal and dendritic opal.

Despite the name, common opal includes many attractive gem materials such as Peruvian blue and pink opal, green prase opal, and Andean opals. In the opal-mining sense, potch also refers to the colorless host opal that surrounds precious opal seams.

It is abundant, affordable, and widely used in cabochons, beads, and carvings.

Common opal forms in the same way as precious opal, by the deposition of hydrated silica from groundwater into cavities, cracks, and porous rocks at low temperatures near the surface. The key difference is internal structure: in common opal the silica spheres are irregular in size or disordered, so they do not diffract light into spectral colors.

It occurs in both volcanic settings (in vesicles and fractures of basalt and rhyolite) and sedimentary settings (in weathered claystones and sandstones). Various trace elements (iron, nickel, copper, manganese) impart its many body colors.

Deposits are found worldwide, including Australia, Peru, Mexico, the United States, and Madagascar.

Look for an opal-textured stone with a solid body color but no flashing rainbow play-of-color, a waxy to vitreous luster, white streak, and Mohs hardness of 5.5-6.5.

Look-alikes: Chalcedony and jasper are harder (around 7) and do not craze. Glass (opalite is often man-made glass) shows bubbles and a uniform appearance. Magnesite or howlite (white) are softer and more chalky.

To confirm opal, note its lower hardness than quartz, its conchoidal fracture, and its tendency to craze when dried. Common opal differs from precious opal simply by the absence of play-of-color.

Common opal is used extensively for cabochons, beads, carvings, and affordable jewelry, with attractive varieties like Peruvian blue, pink, and green opal commanding modest premiums. In opal mining, potch also serves as the natural backing for opal doublets and triplets.

It has minor industrial and ornamental applications and is widely used in tumbled-stone and metaphysical markets, where various colors are linked to different chakras (not a scientific claim).

Like all opal, it can craze if dried out, so finished pieces should avoid heat and prolonged dryness.