This perfect triangle comes from a Chevron Amethyst – an extreme example of color zoning within a single amethyst crystal. Amethyst crystals grow slowly and the composition of the water delivering the iron and the silica needed for crystal growth can change over time. When large amounts of iron are present, the darkest color of amethyst forms; when there is very little iron, the quartz remains white. Later, radiation emitted by minerals in the surrounding rock modify the iron-heavy sections to produce the purple color. Each of these zones records a time interval in the growth of the crystal similar to the growth rings of a tree.