Semiblack Opal

Semiblack Opal, also called dark opal, is precious opal whose body tone falls between true black opal and light opal. On the standard opal body-tone scale it occupies the middle-dark range (roughly N5-N6).

Like all opal it is hydrated silica with play-of-color produced by diffraction. The darker background makes the spectral colors stand out more vividly than they would against a pale body.

It offers much of the visual drama of black opal at a more accessible price, making it popular with both collectors and jewelers.

Semiblack Opal forms exactly like other precious opal: silica-rich groundwater deposits ordered silica spheres in seams and cavities, and play-of-color arises from light diffraction through that lattice.

The darker body tone results from trace impurities and fine inclusions of carbon or iron oxides distributed through the silica, which darken the background without reaching the deep black of true black opal.

Much semiblack opal comes from the Lightning Ridge area of New South Wales, Australia, the same district famous for black opal, as well as other Australian fields.

Judge body tone against an opal tone scale: semiblack sits darker than grey light opal but lighter than true black opal. Play-of-color appears bright against the dark background.

Hardness is 5.5-6.5, streak white, luster vitreous to waxy.

Distinguish solid semiblack opal from doublets and triplets, where a thin opal slice is backed by dark material to mimic a dark body tone; inspect the stone's edge for a glue line and a uniform dark backing layer.

Semiblack Opal is set in rings, pendants, and earrings, valued for delivering strong color contrast at a lower cost than black opal. Brightness, pattern, and color range drive its value.

As a soft, water-bearing gem it needs protection from knocks, heat, chemicals, and drying that can cause crazing.

It shares opal's role as the October birthstone and its associations with creativity and hope. Practically, its appeal is the vivid play-of-color enhanced by a dark body tone.