Ametrine

Ametrine, also known as trystine or by its trade name as bolivianite, is a naturally occurring variety of quartz. It is a mixture of amethyst and citrine with zones of purple and yellow or orange. Almost all commercially available ametrine is mined in Bolivia.

Ametrine
Ametrine Emerald Cut
General
CategorySilicate mineral
Formula
(repeating unit)		Silica (silicon dioxide, SiO2)
Crystal systemHexagonal
Identification
Formula mass60.08 g/mol
ColorPurple , Yellow
Crystal habit6-sided prism ending in 6-sided pyramid (typical)
TwinningDauphine law and Brazil law
Cleavagenone
FractureConchoidal
Mohs scale hardness7
LustreVitreous
Streakwhite
DiaphaneityTransparent to translucent
Specific gravity2.65
Optical propertiesUnixal (+)
Refractive indexnω = 1.543–1.553
nε = 1.552–1.554
Birefringence+0.009
PleochroismAmethyst section shows weak to moderate purple/reddish purple
Citrine section shows very weak yellow/orange [1]
Melting point1650±75 °C
Diagnostic featuresDistinct segments that are purple and yellow
SolubilityInsoluble in common solvents
Common impuritiesIron
Ametrine containing amethyst and citrine, from Bolivia

The colour of the zones visible within ametrine are due to differing oxidation states of iron within the crystal. The citrine segments have oxidized iron while the amethyst segments are unoxidized. The different oxidation states occur due to there being a temperature gradient across the crystal during its formation. Artificial ametrine is created from natural citrine through beta irradiation (which creates an amethyst portion), or from an amethyst that is turned into citrine through differential heat treatment.[2]

Ametrine in the low price segment may stem from synthetic material. Green-yellow or golden-blue ametrine does not exist naturally. This gem only exists in Bolivia, it comes from the Ricón del Tigre area, from the Anahí mine.

Structure

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.

History

Legend has it that ametrine was first introduced to Europe by a conquistador's gifts to the Spanish Queen in the 1600s, after he received a mine in Bolivia as a dowry when he married a princess from the native Ayoreos tribe.[3]

References
  1. "Ametrine Value, Price, and Jewelry Information". International Gem Society. Retrieved 2019-09-14.
  2. "What does Amethyst, Ametrine and Citrine have in common?". Gem Rock Auctions. Retrieved 2019-09-15.
  3. Vasconcelos, Paolo; Wenk, Hanz-Rudolf; Rossman, George. "The Anahí Ametrine Mine, Bolivia," Gems and Gemology, Spring 1994, p. 4-23
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