Sapphire Star.  Sapphire, of the Corundum crystal family, can, if opaque, be cut cabochon (round, without facets) in such a way as to bring to the surface a six-point “star”, or asterism.
Sapphire, when opaque, is opaque because of its dense inclusions of intersecting rutile threads. Light penetrates through the lattice work of these  inclusions just slightly and is reflected, refracted back to the surface in the form of the six-point “star”. This star phenomenon is permanent, is most evident under direct sunlight or pin-point light, and moves across the surface of the gemstone following the light source.
The asterism in the above gemstone is quite sharp.
Colour in Star Sapphires can vary from grey all the way through to Sapphire blue. The colour of the above gemstone is a very pretty mauve-grey. If the above gemstone had been a Sapphire blue, same size and same weight, then the price of this ring would be more than double the above.