Star sapphire is an extremely rare and valuable blue gem found in clusters of sapphires, which are themselves only found in clusters of bauxite.
In Real Life
The "star" pattern that this mineral is known for is caused by rutile (Titanium dioxide: TiO2) crystals that have impregnated the host corundum crystal. Star sapphires are only smoothed into cabochons, as cutting them with facets does not allow the star pattern to be seen.
[SOLID_DENSITY:4005] Common to corundums. Range 3960 - 4050