Ruby is a variety of corundum, which is an oxide of aluminium (Al₂O₃). The red color of ruby comes from the presence of chromium (Cr³⁺) ions in the crystal structure, substituting for some of the aluminium ions.

So, the composition of ruby is primarily aluminium oxide (Al₂O₃) with a small amount of chromium oxide (Cr₂O₃).

The exact amount of chromium oxide can vary, but typically ranges from 0.5% to 3% by weight. This small amount of chromium is enough to impart the characteristic red color to ruby.