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: Ametrine

Ametrine Gemstones used in Jewelry

Rough & Faceted Ametrine Crystals

Source: Bolivia

Birthstone: February (amethyst), November (citrine)

Ametrine (also known as "trystine" or "Bolivianite") is a rare, naturally bicolored variation of amethyst and citrine that is found only in Bolivia. Ametrine has a color transition that is similar to tourmaline, and is caused by dichromatism. Ametrine is a naturally occurring variety of quartz that contains trace elements of Fe3 (citrine yellow) and Fe4 (amethyst purple) iron ions. The only major source for natural Ametrine in the world is from the Anah’ Mine in Santa Cruz, Bolivia.

Ametrine is typically associated with igneous rock, grown in pegmatites and geodes that formed during the Andean mountain building process. Being a variety of quartz, ametrine crystalizes in the rhombohedral crystal system (trigonal-trapezohedral), with an "enantiomorphic" and prismatic crystal habit, forming left-right 'Brazil-law' twins (below, center), v-shaped 'Japan-law' twins (below, right), or 'DauphinŽ-law' twins of 6-sided prisms; each ending in 6-sided pyramids. Like all quartz, ametrine is Piezoelectric.

The purple color in ametrine comes from trace amounts of iron (40 parts per million) that have been oxidized from exposure to naturally occurring ionizing-radiation that is a byproduct of the decay of potassium 40 [4]. The yellow 'citrine' color is created by hydrous iron oxide, and/or exposure to naturally occuring gamma rays.

Ametrine Crystal Structure

Ametrine, amethyst and citrine all belong to the quartz family of minerals, and are classified as tectosilicates. Although quartz is one of the most commonly found minerals within the earth's crust, natural ametrine is extremely rare. The chemical silica dioxide is the principle component in ametrine (quartz), and ametrine is formed from silica rich liquids deposited in gas cavities or encrusted along the walls of geodes within lava.

Ametrine (Quartz) Crystallography, Chemistry, Physical Properties

Crystal System trigonal, hexagonal (trigonal-trapezohedral)
Crystal Habit enantiomorphic, prismatic
Specific gravity (SG) 2.65
Mohs Hardness Scale 7.0 to 7.5
Toughness good
Fracture conchoidal
Cleavage weak in three directions
Streak white
Chemical Composition SiO2 (quartz: silicon dioxide)

Ametrine (Quartz) Optical Properties

Optical Properties double refractive
Refractive Index 1.544 to 1.553
Birefringence +0.009 (B-G interval)
Pleochroism/Dichroism weak dichroism
Surface Luster vitreous (glassy)
Diaphaneity transparent, translucent
Gem Color purple, brown, orangy-yellow

It is rumored that ametrine was first introduced to Europe in the 17th century, by a conquistador named Don Luis Felipe, who received an ametrine mine (the Anahi Mine) as a dowry for marrying Princess Anah’ from Bolivia's Ayoreos tribe [7].

Ametrine Rough

   Fancy Cut Ametrine

Fancy Cut Ametrine Ring

To bring out the "ametrine" color in lightly colored to colorless quartz a combination of irradiation and heating is used. For more exotic colors, gamma irradiation can bring out a greenish-yellow color.

Russian Synthetic Ametrine

Much of the ametrine on the commercial market today is synthetic (hydrothermal created ametrine); being produced at a gem laboratory in Alexandrov, Russia since 1994. Synthetic ametrine was invented at the Institute for Experimental Mineralogy (Chernogolovka Science Center) near Moscow.

Synthetic ametrine has many of the same physical properties as its natural counterpart, making identification difficult. A clear indication of synthetic material is "ametrine" that is a combination of green and yellow or golden-yellow and sky-blue; colors which do not exist naturally.

Artificial or "lab-grown" ametrine is created by growing quartz crystals in a hydrothermal solution, in a pressure vessel known as an autoclave. By exerting varying pressure and/or temperature in the different areas of the autoclave, a feed material (lascas) dissolves in the hotter zone and is redeposited onto seed crystals which are located in the cooler zone; thereby forming synthetic quartz crystals [2, 5]. The entire growth process takes between 30 to 60 days.

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Bibliography and Reference on Ametrine

1. Judith Crowe, The Jeweler's Directory of Gemstones . DK Publishing.

2. GIA, Characteristics of Citrine, Ametrine & Smoky Quartz www.gia.edu

3. ICA, Ametrine: Spinning purple into gold . www.gemstone.org

4. Caltech, Ametrine . minerals.caltech.edu

5. GIA, Russian Synthetic Ametrine . www.gia.edu

6. Antoinette L . Matlins, Antonio C. Bonanno, Gem Identification Made Easy . Gemstone Press

7. MSN, The History of Ametrine . www.groups.msn.com

8. Vasconcelos, Wenk, Rossman, The Anah’ Ametrine Mine, Bolivia. Gems & Gemology, 30, 4-23.

9. A.C. Akhavan, Twinning in Quartz Crystals www.quartzpage.de

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