Take quartz that is part amethyst and part citrine and you get the rare combination material "Ametrine!" In nature, it occurs quite rarely, the finest examples being from the Anahi Mine in the high Andes of Bolivia. The material was briefly common on the market but in the earlier 2000s has dried up considerably and larger stones are becoming harder to source. This stone is untreated and natural - some citrines are heated, for the gem trade (but the hue is distinct). The gem shows superb color saturation in both purple and orange which makes for maximal contrast and quality. The rectangular cut is popular for this material as it highlights the color shift in a piece of finished jewelry. There's a nice 60/40 split in colors, with the emphasis on amethyst. Again, a choice quality of a fairly unique, one-localtion item, that is surprisingly difficult to source today. Cheaper ones are available easily, but this quality is more desirable and so I would pay a little premium to have it.