What Is Agate? Most of Its Bright Color Is Dyed

Agate is a banded variety of chalcedony, and its natural coloring tends toward muted greys, browns and soft blues. It's also porous enough to absorb dye readily, which is exactly why so much commercial agate — including the bright purple and blue "geode slice" coasters and bookends popular in home decor — has been artificially colored. The process typically involves soaking the stone in a chemical dye solution, sometimes combined with heat to help set the color permanently.

Naturally colored agate does exist and tends to be prized specifically for its subtler, more muted banding rather than saturated color. Major sources include Brazil, Uruguay and India, with Brazilian material especially common as the base for dyed decorative slices.