Star rubies are extremely rare red gems found in clusters of rubies, which are themselves only found in clusters of bauxite - among the most valuable gems that can be found in Dwarf Fortress, on par with diamonds.
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 rubies are only smoothed into cabochons, as cutting them with facets does not allow the star pattern to be seen.
[TILE:15][IS_GEM:star ruby:star rubies:OVERWRITE_SOLID][DISPLAY_COLOR:4:7:1][MATERIAL_VALUE:60]
[SOLID_DENSITY:4005] Common to corundums. Range 3960 - 4050