Corundum of a red colour are identified as rubies. Being corundum these have all the optical, physical and chemical properties of the mineral.

These also have negligible quantities of chromic oxide in their compositions which do not in any way affect the general chemical formula. It is this element, chromic oxide which is responsible for the red colour of ruby.

Mineral Ruby
Chemistry Al2O3
Refractive Index 1.762 to 1.770
Birefringence 0.008 to 0.010
Specific Gravity  4.00 (+/- 0.05)
Mohs Hardness 9