Ametrine gemstone beads color formation

Ametrine contains both amethyst and citrine in contact with one another.  The colors of amethyst and citrine are produced by iron impurities with different oxidation states within the quartz [1].  Purple is thought to be produced by Fe⁴⁺ related color centers, of which the Fe impurities are oxidized to Fe⁴⁺ by natural radiation emitted in nearby rocks. The golden-yellow is thought to be produced by Fe³⁺ related color centers. The different oxidation states occur due to there being a temperature gradient across the crystal during its formation and by natural radiation.  The concentration of iron is much higher in the yellow than in the purple sectors. If heating up a natural Ametrine, only the purple sectors loose their color, and the result is a crystal with 3 almost colorless and 3 yellow sectors.

If a well-formed Ametrine crystal is sawn perpendicular to the c-axis, the color zones of amethyst and citrine often form a geometric pattern that radiates outwards from the c-axis like the pieces of a pie. Straight-line contacts separate zones of amethyst from zones of citrine.  These crystals usually contain zones of clear quartz, amethyst and citrine. When these crystals are cut into pieces that are appropriately sized for faceting gemstones, only a portion of the stones will be Ametrine. The remainder will be amethyst, citrine, and clear quartz [2].

Laboratory experiments in 1981 determined that heat and irradiation can be used to convert natural amethyst into a bicolor material that has an appearance similar to natural Ametrine [4].  Since 1994, a Russian laboratory has perfected the industrial production of bicolored quartz crystals that are later irradiated to bring out the typical Ametrine colors [3] to form Ametrine gemstone beads for jewelry.

Like all crystal quartzes, the Ametrine has a hardness of 7 on the Mohs scale and is thus insensitive to scratches.

Ametrine is rarely found in nature. The only known and well confirmed location for Ametrine is an area in Bolivia near the Brazilian border [2]. It's commercially exploited and almost all of the world's commercial natural Ametrine production has been from the Anahi  Mine in southeastern Bolivia.

High quality Ametrine is usually faceted for jewelry and less quality Ametrine is most often used as stone beads with different shapes:  round, coin, oval, square, pillow, cube, tube, et al. With Ametrine beads in your favorite color and shape, you even can create your own unique piece of Ametrine jewelry, and with plenty of opportunity to customize to fit your unique style. There are plenty of low-cost Ametrine beads available through online store and be sure to check out the discounted Ametrine beads first to start your design.

Reference:

[1] G.R. Rossman, “Colored varieties of the silica minerals”, Reviews in Mineralogy, Vol.29 

[2] Vasconcelos P, Wenk HR, Rossman GR (1994) The Anahí ametrine mine,
Bolivia. Gems and Gemology 30, 4-23.

[3] Balitsky VS, Lu T, Rossman GR, Makhina IB, Mar'in, AA, Shigley JE, Elen S,
Dorogovin BA (1999) Russian synthetic ametrine. Gems and Gemology 35,
122-134.

[4] Balitsky, V. S.; Machina, I. B.; Mar'in, A. A.; Shigley, J. E.; Rossman, G. R.; Lu, T. (2000): Industrial growth, morphology and some properties of Bi-colored amethyst-citrine quartz (ametrine). Journal of Crystal Growth 212, 255-260.