Australian Black Sapphire, oval cabochon.
This is a genuine Corundum Sapphire gemstone. It is black and opaque due to its countless rutile rod inclusions lined up in straight lines, so densely arrayed as to render the gemstone quite opaque; light penetrating is instantly refracted back, creating a “Star” across the gemstone surface. Though the startling effect is not captured in the images, this gemstone has been cut to bring out this six-point “Star”, or “Asterism”, especially under pin-point light or direct sunlight, and this asterism is permanent. It is a “Star Sapphire”
9mm width across the top, 6mm at rear and 2mm thick, a strong durable ring. And the Black Star Sapphire itself is very durable, being of the extremely tough Corundum, the second toughest crystal gemstone after Diamond, and being cut cabochon, having no facets to knock and nick.

Note: the above Australian Black Sapphire ring is completely handmade, in Sydney, not cast from a wax model. It takes a bench jeweller of considerable craft to fashion by hand a ring of Silver and Gold as substantial as this.
Note also: 2.8 grams of solid 9ct Gold used in the mount for the Sapphire cabochon. The last image in the above image gallery shows the thickness of the Gold used in this Sydney-handcrafted setting.