Ametrine: Crystals, Properties, Occurrence

Ametrine is a naturally occurring variety of quartz that is a bicolor gemstone, displaying bands of both amethyst purple and citrine yellow. Ametrine is a relatively rare gemstone, and the finest specimens are highly prized by collectors and jewelry lovers, ametrine occurs naturally only at the Anahí Mine in the Sandoval Province of eastern Bolivia.

Ametrine is composed of silicon dioxide (SiO₂ ) and it is a tectosilicate, which means it has a silicate framework linked together through shared oxygen atoms.

Ametrine is a mix of both amethyst and Citrine producing a mixture of purple and yellow/orange crystal colours. It is produced under such exceptional and improbable conditions, requiring a perfect combination of iron presence and differing temperatures within a very confined area, that it has occurred only once that we know of, in only one known place in the world: Santa Cruz, Bolivia.

Ametrine: A Blend of Amethyst and Citrine

Rough Ametrine from Bolivia.
Credit: GoldenHourMinerals

Ametrine Formation

The purple and yellow-orange colors in ametrine are attributed to the presence of trace elements within the quartz crystal. The purple color is primarily due to iron (Fe) impurities, while the yellow-orange color is attributed to manganese (Mn) impurities.

There are two main theories about how ametrine is formed:

Varying Iron Concentrations: The first theory suggests that the varying concentrations of iron in the quartz crystal cause the different colors. As the concentration of iron increases, the quartz becomes more purple, and as the concentration of iron decreases, the quartz becomes more orange.

Varying Oxidation States of Iron: The second theory suggests that the different colors of ametrine are caused by the varying oxidation states of iron in the quartz crystal. When iron is in its oxidized state (Fe³⁺), the quartz becomes purple, and when iron is in its reduced state (Fe²⁺), the quartz becomes orange.

Factors Affecting Ametrine Color Distribution

The distribution of purple and yellow-orange colors in ametrine crystals is influenced by several factors, including:

Temperature: The temperature of the hydrothermal fluids plays a crucial role in determining the color of the quartz crystals that form. Higher temperatures favor the formation of citrine, while lower temperatures favor the formation of amethyst.
Pressure: Pressure also affects the color of quartz crystals, with lower pressure favoring the formation of citrine and higher pressure favoring the formation of amethyst.
Impurity Concentration: The concentration of impurities, such as iron oxide and aluminum oxide, also influences the color of the quartz crystals. Higher concentrations of iron oxide lead to deeper purple hues, while higher concentrations of aluminum oxide lead to more vibrant yellow hues.
Growth Conditions: The specific conditions under which the quartz crystals grow, such as the rate of cooling and the presence of other minerals, can also influence the color zoning and patterns of ametrine.

Uncut Ametrine crystal from Bolivia.

Ametrine Properties

Ametrine is a stunning semi-precious gemstone known for its unique bicoloration, blending the vibrant violet shades of amethyst with the sunny yellow hues of citrine. This gemstone belongs to the quartz group and shares many of its physical properties. Let's delve into the details:

Composition: Primarily SiO₂ (Silicon Dioxide) - Same as Amethyst and Citrine, both considered varieties of Quartz. with trace amounts of iron and manganese, which contribute to its color.

Ametrine Physical Properties

Color: Main colors: Purple and Yellow, often banded or zoned within the same crystal.
Color variations: The intensity and distribution of the purple and yellow can vary greatly, from gems with bold, contrasting colors to those with subtle, blended hues. Some ametrines may also display green or orange flashes.
Crystal System: Trigonal
Luster: Vitreous - Similar to glass, with a bright, glassy shine.
Streak: White - Like most quartz varieties, ametrine leaves a white streak when scratched on a rough surface.
Hardness: 7 on the Mohs scale - This makes ametrine relatively durable and scratch-resistant, suitable for everyday jewelry wear.
Cleavage: None - Ametrine lacks perfect cleavage, meaning it doesn't break predictably along specific planes.
Crystal Form: Prismatic - Ametrine commonly forms in six-sided prismatic crystals, sometimes with pyramidal terminations.
Density: 2.65 g/cm³ - Slightly denser than water, similar to other quartz varieties.
Transparency: Transparent to translucent - The degree of transparency can vary depending on the crystal's clarity and inclusions.
Fracture: Conchoidal - When broken, ametrine fractures with smooth, curved surfaces like a seashell.
Solubility: Insoluble in most common solvents, including water and acids.
Magnetism: Non-magnetic.

Ametrine

Ametrine Optical Properties

Fluorescence: May exhibit weak to moderate fluorescence under long-wave ultraviolet light, appearing yellow or orange.
Pleochroism: Weak to distinct - Ametrine can exhibit slight color variations depending on the viewing angle. The purple and yellow hues may appear more intense in certain directions.
Refractive Index: 1.54 - 1.55 - This is a measure of how light bends when passing through the gemstone, and it can be used to help identify ametrine.

Occurrence of Ametrine

Anahi Mine, Bolivia: The Anahi Mine in Bolivia is considered the world's primary source of ametrine. The mine is located in the Potosí Department and produces ametrine crystals of exceptional color and clarity. The Anahi ametrine crystals range in size from 10 to 30 cm in length and 4 to 12 cm in diameter. When sliced and polished, the interior of the crystals show the typical color zoning. The amethyst and citrine zones run from top to bottom, parallel to the crystal’s c-axis.

Rough Ametrine from Bolivia : A Blend of Amethyst and Citrine

Rough Ametrine Crystal from Bolivia

Facts about Ametrine

The color zoning in ametrine is caused by a temperature gradient across the crystal during its formation.
Ametrine is often cut into cabochons or other shapes that showcase its unique appearance.
Ametrine is a popular choice for jewelry, especially earrings, necklaces, and bracelets.

Ametrine Gemstone

Ametrine is a highly sought-after gemstone due to its unique combination of purple and yellow colors. It is often used in jewelry making, where its beauty and rarity make it a truly special gemstone. Ametrine is also believed to possess certain metaphysical properties, making it appealing to those who believe in crystal healing.