The Geology of Color

Color in minerals comes from three main mechanisms. Transition metal impurities are the most common - atoms of iron, copper, chromium, or manganese substituting into a crystal's structure absorb specific wavelengths of light and transmit the rest as visible color.

Crystal field effects determine exactly which wavelengths are absorbed - the same element can produce different colors in different mineral structures. Chromium makes ruby red and emerald green because the surrounding crystal geometry changes which wavelengths chromium absorbs.

Physical optics produce color without any chromophore element at all. Opal's play of color comes from diffraction. Labradorite's flash comes from thin-film interference. Blue lace agate's blue comes from Rayleigh scattering. These are the same physics that make rainbows, soap bubbles, and the sky blue.