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.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Is a Variety of||Quartz|
|Refractive Index||1.544 - 1.553|
|Special Care Instructions||None|
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.
Quartz, at hardness 7 with no cleavages, make 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.
The value of ametrine is rather modest. Like most quartz gems it is often found in fairly large, clean pieces, so the per carat price increase 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.