Ametrine
Ametrine is a variety of quartz that contains both amethyst and citrine sectors in the same crystal. Both amethyst and citrine are colored by small amounts of iron (approx. 40 parts per million). Amethyst color develops when iron-containing quartz is exposed to ionizing radiation. In nature, gamma rays from the decay of potassium-40 are the most likely source of ionizing radiation. The model currently accepted is that radiation oxidizes Fe3+ to Fe4+. There is still uncertainty about the site of the iron. Both interstitial sites in the c-axis channels, and the silicon tetrahedral sites have been proposed as the site of the amethyst center. Citrine color is from Fe3+. The properties of the Fe3+ spectra suggest that the Fe3+ ions are aggregated and hydrated in clusters of unknown size.
Natural Occurrences
Bolivia
The only significant source of natural ametrine is the Anahà mine, in eastern Bolivia. The mine is operated by Minerales y Metales del Oriente, S.R.L., Santa Cruz, Bolivia. It is the source of all the natural ametrine currently on the world's market. In the early days of production, there was much mis-information about the locality with Brazil and Uruguay frequently being mentioned as the source of ametrine.
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