Ametrine is a rare, naturally occurring bi-colored quartz variety that uniquely combines the purple hue of amethyst and the yellow-orange color of citrine within a single crystal. This distinct two-toned appearance makes it a highly appealing and sought-after gemstone for jewelry and collectors. The name is a portmanteau, reflecting the blend of the two quartz varieties it contains. Natural ametrine is found in large, clean crystals, and its value is primarily dependent on the vibrancy and distinct separation of its two colors.
The World’s Exclusive Primary Source
The question of where ametrine is found has a remarkably specific answer, as virtually all commercially available natural material originates from a single location. That sole source is the Anahí Mine, situated in the remote tropical lowlands of southeastern Bolivia, near the border with Brazil. This mine is the world’s only recognized commercial producer of gem-quality ametrine, giving Bolivia a near-total monopoly on the natural supply.
The mine’s association with the gem dates back to the 17th century, when a Spanish conquistador reportedly received the mine as a dowry after marrying a native princess named Anahí. This historical lore explains the mine’s name and the alternative trade name for the stone, Bolivianite. After being lost for centuries, the mine was rediscovered and began appearing on the international market in the 1970s.
The Anahí Mine exploits quartz veins found within a dolomitic limestone formation. While the mine is most famous for ametrine, its production also includes significant amounts of natural amethyst and citrine, the components of the bi-color gem. Ametrine remains the most lucrative and defining product of the operation.
Geological Requirements for Formation
Like amethyst and citrine, the color in ametrine is caused by trace amounts of iron impurities within the silicon dioxide crystal structure. The difference in color is due to the iron existing in varying oxidation states within the crystal lattice.
The most crucial requirement is a rare and precise temperature gradient during the crystal’s growth, which causes differential heating across the crystal face. This gradient leads to one side of the crystal being slightly hotter than the other, which oxidizes the iron and creates the yellow citrine color in that zone. The cooler side of the crystal remains as the purple amethyst.
The amethyst zone’s purple color is finalized when the iron-containing quartz is exposed to natural ionizing radiation, likely gamma rays from the decay of potassium-40 in the surrounding rock. The combination of this radiation and the precise temperature difference allows the sharp color boundary to form, where the two color zones meet along the crystallographic planes. This exact interplay of iron presence, temperature variation, and differential natural irradiation is what makes the Anahí deposit geologically exceptional.
Secondary Deposits and Identifying Authenticity
While Bolivia’s Anahí Mine is the only commercial source, minor, non-commercial deposits have been reported in a few other locations, such as Brazil and India. These finds are typically small, sporadic, and do not contribute meaningfully to the global gem market. The availability of treated or synthetic materials is widespread, and they are often marketed as genuine.
The most common non-natural ametrine is created by partially heating and then irradiating natural amethyst to induce the bi-color effect. Synthetic ametrine has been available since the 1990s and can be difficult to distinguish from its natural counterpart.
Natural ametrine often contains fluid inclusions, which are tiny pockets of the liquid from which the crystal grew, providing a fingerprint of its natural formation. In contrast, the color boundary in synthetic material is often oriented at an oblique angle to the crystal’s optical axis, a feature that trained gemologists can use for identification. Consumers should be wary of stones with blurred color transitions or unusual hues like green or blue, as these colors do not occur naturally in ametrine.