Ametrine is a single quartz crystal that holds both purple amethyst and yellow to orange citrine in separate zones. Nearly all natural ametrine on the market comes from one area, the Anahi mine in eastern Bolivia. The name blends "amethyst" and "citrine," and cutters orient each stone so that both colors show in the finished gem.
What ametrine is
Ametrine is ordinary quartz, silicon dioxide, with the same hardness (Mohs 7), refractive index (about 1.544 to 1.553) and specific gravity (2.65) as amethyst and citrine. It is a trade name for a color variety, so you will not find it listed as a separate mineral. The two colors meet along fairly sharp boundaries. In a well-cut stone the purple and the gold sit side by side; in others, the zones blend into a peachy or smoky middle.
Both colors trace back to iron. In amethyst, iron combines with damage from natural radiation to form color centers that absorb yellow-green light. In citrine, iron in a different state gives yellow to orange tones. Ametrine has both in one crystal, divided by the way the crystal grew. The color in gemstones page covers color centers and trace-element coloring in more depth.
How the color zones form
Quartz crystals grow outward from a base, adding layer after layer on their faces. The pointed end of a quartz crystal has two sets of sloping faces, the major and minor rhombohedra. Each set of faces builds its own wedge-shaped region inside the crystal, called a growth sector. Different sectors can take up trace elements in different amounts and in different chemical states.
In Bolivian ametrine, the purple and yellow colors usually follow these sectors. One set takes in iron in a way that later forms amethyst color centers after natural irradiation. The neighboring set takes in iron in a form that stays yellow. Researchers have linked the split to temperature differences in the fluids during growth, which can affect the iron in each sector differently. Ametrine crystals also commonly show Brazil-law twinning, where right- and left-handed quartz are intergrown in thin lamellae. That twinning is common in amethyst from many sources and shows up as stripes when a stone is viewed between crossed polarizers.
Seen down the long axis of a crystal, the result often looks like a pie divided into alternating purple and yellow slices. Cut across the side, the same crystal shows bands of color running toward the tip.
The Anahi mine and Bolivian ametrine
The Anahi mine lies in the Santa Cruz department of eastern Bolivia, close to the border with Brazil, in a remote lowland area. The quartz formed in cavities in limestone and dolomite rock, where mineral-rich fluids deposited crystals.
Local tradition tells that a Spanish conquistador received the mine as a dowry in the seventeenth century when he married a princess of the Ayoreo people named Anahi. The story is a legend and cannot be verified, but the mine still carries her name. Ametrine reached the international gem trade in larger quantities in the 1970s and 1980s, when Bolivian material began appearing at gem shows, and it has been a fixture of colored stone jewelry since.
Small amounts of bicolor amethyst-citrine quartz have been reported from other places, including Brazil and India, but none has matched Bolivia's output or color. A gem sold as natural ametrine without a stated source is very likely Bolivian. If you are curious about other South American gem regions, see gems of Minas Gerais.
Cutting for the color split
Cutting ametrine is mostly a matter of orientation. A cutter studies the rough to find where the boundary lies, then chooses a shape that shows both colors in good balance.
- Long rectangular step cuts and emerald cuts are the classic choice. The cutter sets the boundary across the middle so one half is purple and the other gold.
- Concave cuts, carvings and fantasy cuts can show sharp lines of color or several alternating bands. Gem carvers in Germany and elsewhere have used ametrine for this kind of work, as described in Idar-Oberstein gem cutting.
- Checkerboard tops and round brilliants tend to mix the colors. Light bouncing through many facets blends purple and gold into a peach or pinkish tone, which some buyers like and others avoid.
A good ametrine has a clear contrast between the two colors with no muddy brown zone at the boundary. Most buyers look for roughly equal amounts of each color, although stones weighted toward purple are popular too. Ametrine cuts well into large sizes, and stones of 10 carats and more are not unusual. The gemstone cut shapes and styles page explains the facet styles mentioned here.
Synthetic and treated ametrine
Synthetic ametrine grown by the hydrothermal method has been made in Russia since the 1990s. It has the same composition and physical properties as natural ametrine. Gemologists separate the two with a polarizing microscope and spectroscopy, looking for the twinning pattern and growth features of natural crystals. Some synthetics show unnatural, very straight color zones or lack the typical fluid inclusions.
Ametrine can also be produced by treating natural amethyst. Heating part of an amethyst crystal can turn some zones yellow while leaving others purple, and irradiation of some synthetic quartz has given similar bicolor effects. Treated or synthetic material should always be disclosed. See gemstone treatments explained for the methods and how sellers are expected to describe them.
If a seller offers large, flawless ametrine at a very low price, ask whether it is natural and whether a lab report is available. A gem lab with infrared spectroscopy and a polariscope can usually settle the question.
Wear and care
Ametrine wears like other quartz. At Mohs 7 it handles daily wear in rings, though a high-set stone can still chip if knocked hard. It has no cleavage, which makes it fairly tough.
Like amethyst, the purple zone can fade with long exposure to strong light or heat. Store ametrine jewelry out of direct sun, and ask jewelers to keep it away from torch heat during repairs. Clean with warm soapy water and a soft brush. Ultrasonic cleaning is usually safe for clean stones but risky for heavily included ones, and steam is best avoided. More detail is in the jewelry cleaning guide.
Frequently asked questions
Is ametrine natural?
Natural ametrine exists, and most of it comes from the Anahi mine in Bolivia. Synthetic and treated ametrine also exist, so a lab report helps for important purchases.
Is ametrine a birthstone?
Ametrine is not on the official modern birthstone list. Because it combines amethyst (February) and citrine (November), some buyers choose it to represent both months.
Why does my ametrine look peach instead of purple and yellow?
Faceting styles with many small facets mix the two colors as light bounces inside the stone. Step cuts and emerald cuts keep the colors separate.
How hard is ametrine?
Ametrine is quartz and rates 7 on the Mohs scale. It resists scratches from most household dust and suits everyday jewelry with reasonable care.