A star sapphire is a type of sapphire that exhibits a starlike phenomenon known as asterism; blue stones are known as “star sapphire .

Star sapphires contain intersecting needle-like inclusions following the underlying crystal structure that causes the appearance of a six-rayed “star”-shaped pattern when viewed with a single overhead light source. The inclusion is often the mineral rutile, a mineral composed primarily of titanium dioxide

Mineral Star Sapphire
Chemistry Al2O3
Refractive Index 1.762 to 1.770
Birefringence 0.008 to 0.010
Specific Gravity  4.00
Mohs Hardness 9