Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

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January, 2002

Ametrine

Quartz which occurs in bands of yellow and purple has been given the name of Ametrine (amethyst + citrine). Originally discovered in Brazil, the world's current supply comes from one area in Bolivia. The crystals from this mine often exhibit an abrupt color transition, which probably reflects dramatic changes in temperature during their formation. Much citrine today is produced by heating amethyst, so it is easy to imagine natural heating and/or cooling occurring in such a way as to produce the bicolored quartz. Clarity and good size make it a favored material of gem carvers and cabochon artists as well.

Quartzes at hardness 7 with no cleavages, make rather good jewelry gems, although daily wear in rings will result in eventual dulling of the polish. No special care is required as they are not sensitive to temperature change or household chemicals.

Both heat enhanced natural quartz, and synthetic ametrine are on the market and as they are optically and physically like Nature's product, sophisticated gemological testing is necessary to detect them.

Initially cutters favored windowed emerald shapes with a 50/50 split of colors, and much of the rough is still cut this way. More recently, however; some cutters have begun to cut a variety of shapes, many of which create internal reflections that blend the yellow and purple into attractive shades of rosy gold and mauve, or create mosaic-like flashes of both yellow and purple.

Value Factors

The value of ametrine is rather modest and like most quartz gems it is often found in fairly large, rather clean pieces so the exponential per carat price increase seen in many gems at larger sizes does not occur. The major value point to be considered in the material itself is the depth and vividness of the colors and in many cases how distinct the separation is. Much of the value in many pieces comes from the artistry of the cutting or carving. There is a world of difference to be seen in a commercial grade or native emerald cut, and a fine custom stone, even from the same material.

Gemological Properties:

Makeup: Silicon Dioxide

Hardness: 7

Dispersion: .013

Toughness: Good

Crystal System: Trigonal

Luster: Vitreous

Density: 2/65

RI: 1.54 - 1.55

Cleavage: none

Stones Currently Available:

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