Chocolate Opal

Chocolate opal is precious opal with a rich brown to coffee-colored body tone against which play-of-color flashes appear especially warm and vivid. The dark background, like that of black opal, increases the contrast and brilliance of the spectral colors.

The most famous chocolate opal comes from Ethiopia (Welo and earlier fields) and from Mexico. Ethiopian chocolate opal is often hydrophane, meaning it can absorb water and temporarily change transparency.

It offers a distinctive, earthy alternative to white and black opal and has become popular among collectors and designers.

Chocolate opal forms by deposition of hydrated silica from groundwater in volcanic host rocks. In Ethiopia, the opal occurs within Tertiary volcanic ash and rhyolite layers of the highlands, where silica-rich solutions filled cavities and seams.

The brown body color is attributed to iron oxide impurities incorporated into the silica. Where the silica spheres are uniformly ordered, the opal shows play-of-color; the brown matrix provides the dark backing.

Mexican chocolate opal forms similarly in rhyolitic volcanic settings. As with all opal, low-temperature near-surface conditions and a steady silica supply are required.

Look for a warm brown body color with rainbow play-of-color, a waxy to vitreous luster, white streak, and Mohs hardness of 5.5-6.5. Ethiopian chocolate opal is often hydrophane: a drop of water may make it more translucent (do this test cautiously, as wetting can affect the stone).

Look-alikes: Boulder opal has opal on ironstone rather than a uniform brown opal body. Brown common opal lacks play-of-color. Dyed or smoke-treated opal can mimic the brown tone; suspicious uniformity or concentration of color in cracks suggests treatment.

The combination of brown body and genuine play-of-color is characteristic.

Chocolate opal is cut into cabochons for rings, pendants, and earrings, valued for the warm glow of its play-of-color against the brown body. Ethiopian material is widely available and affordable; fine Mexican chocolate opal is rarer.

Because Ethiopian chocolate opal is often hydrophane, it should be kept away from water, oils, and chemicals that can be absorbed and discolor it, and protected from heat and drying like all opal.

Metaphysically it is linked to grounding and stability (not a scientific claim). It is an increasingly fashionable opal in contemporary jewelry.