Ametrine is a single crystal with colour zones of purple and yellow, it contains both amethyst and citrine; hence the name. With natural ametrine, colour zoning occurs during the crystals growth, due to a temperature gradient in the surrounding fluids from which the crystal obtains it minerals. The temperature gradient causes iron impurities in the crystal to develop into different oxidation states. It is the difference in these iron oxidation states that is responsible for the colour of each zone.
As most commercial citrine is formed by heat-treating amethyst, a naturally formed ametrine crystal is a good example of that process before completion, showing both the before and after effects in one stone.
Naturally coloured ametrine is sourced from the Anahi Mine in Bolivia.
Differential heat-treatment of amethyst is used to produce a citrine zone; irradiation is also used.