Amethyst and citrine are varieties of quartz coloured by iron. They only occur mixed together like this in quantity at one mine in Bolivia, though a few specimens have come out of India and Brazil. The colours are due to the different electronic valencies of iron, related to its oxidation state and are caused by a temperature gradient across the crystal during formation. Amethyst’s purple hue is caused by natural radiation creating colour centre in the crystal and turned into citrine by heat. Most citrine on the market is in fact amethyst that has been roasted in an oven to change the oxidation state of the iron content, and hence the colour.
Both synthetic hydrothermal (grown from aqueous solutions on seed crystals) ametrine from Russia and treated amethyst differentially heated to produce the bicolour effect are also available on the market. Cheap stones are almost certainly synthetic, and non natural combinations such as green and yellow or golden and blue have also been manufactured. Synthetics will sometimes display areas of the colourless seed crystal on which they were grown, but these are usually cut away, especially in high grade material.
The specimen in teh photo weighs 34 carats and was carved by master lapidary Arthur Lee Anderson.
Image credit: Gordon Aatlo.