Black Matrix Opal

Andamooka lies a little over 500 km north-northwest of Adelaide, South Australia. There, matrix opal formed where opal partly replaced limestone boulders and filled pore spaces in sandstone and claystone; in Australia this kind of matrix opal appears to be restricted to the Andamooka fields. Since the mid-1950s it has been darkened by a sugar-acid process: the prepolished stone is soaked in an acidified sugar solution, heated, then heated in concentrated sulfuric acid, which reduces the sugar to black carbon in the pores. Variants without acid, using sugar and baking, are also used. Natural black matrix opal in black basalt is reported from Nicaragua, and black opal matrix from Honduras is sold as "Black Honduran Opal".

Under a 10x loupe, treated Andamooka matrix opal shows a granular groundmass with black carbon particles around patches of precious opal and filling voids and fractures. Compared with Lightning Ridge black opal, it lacks the velvety polish, has an opaque, uniformly black body (natural black opal is usually dark blue, green, brown or gray) and shows patchy to spotty play-of-color. A chip may reveal the original whitish matrix beneath the thin black layer. The Japanese retailer Pascle notes that black matrix opal is often confused with black opal, and that depending on the proportion of opal and sandstone it may be classed as opal or as opal-bearing sandstone.

Avoid damage to the surface: the carbon impregnation is often no deeper than 1 mm, and repolishing may remove the black color. Treated stones should also be protected from strong heat and harsh chemicals, like other opal, and cleaned with a damp cloth rather than in an ultrasonic cleaner.

The Japanese retailer Pascle gives black matrix opal the meanings it associates with opal in general, saying it is traditionally believed to encourage creativity, and presents it as a charm for success at work. Pascle lists it among October birthstones in Japan. These are retail traditions, not verified properties.