Sapphires Black 9x7mm ovals. Australian origin, genuine Corundum Sapphire, Corundum being the crystal family of both Sapphire and Ruby. Black Sapphire has exactly the same chemical composition as Blue Sapphire, but has a much higher concentration of certain trace elements such as iron and titanium. It is opaque and absorbs light, showing as black.
Black Sapphire can be cut cabochon, domed, as the above matching pair, and has a distinct advantage over other black gemstones in that it is Sapphire tough, one of the really hard gemstones, and is difficult to damage. Once polished it tends to simply stay polished.
Cabochon Black Sapphire can be cut in such a way, with the natural needle-like inclusions lined up just so, that light penetrates just enough into these needle-like inclusions to be refracted back out across the surface in the shape of a Star; this is the “asterism” phenomenon.
The above two gemstones display this “asterism”, the star moving across the surface in relation to the light source. Sunlight or pin-point light best brings out the star.