What Is Carnelian? The Chemistry Behind Its Orange-Red

Carnelian is a variety of chalcedony — a microcrystalline form of quartz made up of crystals too small to see individually — colored orange to red-brown by iron oxide impurities. Its color depends on exactly how that iron is distributed and oxidized within the stone.

Heat-treating carnelian to intensify and even out its color isn't a modern practice: there's archaeological evidence that ancient Egyptian and Roman artisans heated the stone, sometimes simply by leaving it in the sun, for the same effect people use ovens for today. Major sources now include India, Brazil and Uruguay.