Asterism (gemology)

In the red instance stellate reflection is rarer; the star-ruby occasionally found with the star-sapphire in Sri Lanka is among the most valued of "fancy stones".

Asterism is generated by reflections of light from twin-lamellae or from extremely fine needle-shaped acicular inclusions within the stone's crystal structure.

[2] The star-effect or "asterism" is caused by the difference in refractive index between the host material and that of the dense inclusions of tiny fibers of rutile (also known as "silk").

In black star sapphire hematite needles formed parallel to the faces of the second order prism produce asterism.

[1] Pliny the Elder's example is consistent with a moonstone; he described it as a colourless stone from India within which was the appearance of a star shining with the light of the moon.