Stone Story
Corundum is colorless in its pure form; ruby's red is produced by chromium substituting for aluminum in the crystal structure. The chromium absorbs certain wavelengths and transmits red, with fluorescence from the same element adding a glow that distinguishes ruby's red from other red gemstones. The same chromium chemistry produces different color outcomes in different mineral hosts: red in spinel and ruby, green in emerald, color-change in alexandrite, making it one of the most consequential trace elements in gemstone color science.