Ametrine

Ametrine is a naturally bicolored quartz that displays both amethyst (purple) and citrine (yellow) colors in the same crystal. It's one of the most visually striking members of the quartz family, and the science behind its two-toned appearance is elegant.

Both colors come from iron, but in different oxidation states. In the purple zones, iron is in the Fe³⁺ state and has been affected by natural gamma radiation to create amethyst color centers. In the yellow zones, the iron is in a different oxidation state that produces the citrine color. The boundary between the two zones can be sharp or gradational.

The Anahi Mine in eastern Bolivia is essentially the world's only commercial source. According to local legend, the mine was given to a Spanish conquistador as a dowry when he married a princess of the Ayoreos tribe named Anahi. The mine was lost for centuries before being rediscovered in the 1960s.

The color distribution in ametrine follows the underlying crystal symmetry - the boundaries between purple and yellow zones align with specific crystallographic directions, which is why skilled gem cutters can orient the stone to show a clean, dramatic color split.

Ametrine is identified by its distinctive bicolor appearance - purple and yellow in the same transparent stone. At hardness 7, it's durable and takes an excellent polish. The color boundary may be sharp (most desirable for gems) or gradual.

Distinguish from synthetic ametrine (exists but requires lab testing), color-zoned amethyst (may show pale-to-dark purple zoning but not yellow), and glass (no color zoning follows crystal symmetry, may have bubbles). The way the color boundary follows crystallographic planes is diagnostic of natural material.