Opal

Opal is a hydrated form of silica containing up to 20% water, and it is amorphous — it has no true crystal structure, classing it as a mineraloid. Its fame rests on play-of-color: flashing rainbow flecks that shift as the stone moves.

This effect comes from a regular internal arrangement of microscopic silica spheres that diffract light. 'Precious opal' shows play-of-color; 'common opal' (potch) does not. Body color varies from milky white and crystal-clear to dark grey and prized black opal, plus the translucent orange-red fire opal.

Opal is the birthstone of October and a national gemstone of Australia, which produces most of the world's supply.

Opal forms at low temperatures near the Earth's surface, where silica-rich water seeps through cracks, cavities, and porous rock and slowly deposits gel-like silica as the water evaporates. Over long periods this silica hardens into opal.

In Australia's great deposits (Coober Pedy, Lightning Ridge, Andamooka), opal filled voids in ancient weathered sedimentary rock and even replaced fossils and wood. Volcanic opal, including Ethiopian and Mexican fire opal, forms in cavities in lava.

Other sources include Brazil, Mexico, the western USA, and Ethiopia, whose Welo opals have become major in the market.

The hallmark is play-of-color — internal flashes of spectral color that move with the stone (different from the surface sheen of opalescence). Opal has a vitreous-to-resinous luster, conchoidal fracture, hardness around 5.5-6.5, and a relatively light weight.

It is often translucent and can be sensitive to drying and cracking (crazing).

Look-alikes: Glass and plastic imitations and 'opalite' (man-made glass) lack true diffraction play-of-color, showing only a milky blue glow. Labradorite shows flash but is a feldspar that is harder and crystalline. Doublets and triplets (thin opal glued to backing) are common and should be checked from the side.

Opal is almost entirely a gemstone, cut into cabochons and set in rings, pendants, and earrings. Fine black opal from Lightning Ridge and bright crystal and boulder opals command very high prices, while common opal is used for beads and carvings.

Because it is relatively soft and can craze, opal jewelry is best protected from knocks, heat, and prolonged dryness.

As October's birthstone, opal carries long-standing symbolism of hope, creativity, and inspiration, and it features prominently in metaphysical practice. Opalized fossils are also scientifically and commercially valuable.