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   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. Citrine color in ametrine comes from the incorporation is small aggregates of Fe3+ most likely in the form of a hydrous iron oxide.    The only significant source of natural ametrine is the Anahí mine, in eastern Bolivia. The mine is operated by Minerales y Metales del Oriente, S.R.L., Santa Cruz, Bolivia, and employs about 70 workers at the mine site. 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.   As is the case for all amethyst, the amethyst color center in ametrine is somewhat photosensitive and will be lost upon prolonged exposure to bright light.  Ametrine is an excellent gemstone for jewelry. 
Finished ametrine jewelry can be viewed here.  It is very durable and rated at 7 on the mohs scale of hardness.   

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