Green-Blue on Brown Semi-Black Opal from Lightning Ridge, Freeform

About Semi-Black Opal

Semi-black opal is a trade description for opal with a noticeably dark grey to charcoal body tone, but not the deepest black body tone used for some black opals. The background may show flashes of blue, green, yellow, orange or red play-of-colour. It is generally opaque to translucent, and its shifting colours arise from light diffraction in regularly arranged silica spheres. Opal is hydrated, non-crystalline silica rather…