A ruby is a variety of the mineral corundum, which is aluminum oxide (Al₂O₃). However, what gives ruby its distinctive red color is the presence of a small amount of chromium (Cr³⁺) ions within the crystal structure.

Here's a breakdown:

* Main component: Aluminum oxide (Al₂O₃)

* Coloring agent: Chromium (Cr³⁺)

The amount of chromium present determines the intensity of the red color. The more chromium, the deeper the red.