Ametrine is a bi-colored quartz: part amethyst and part citrine. It occurs in eastern Bolivia near the border with Brazil.
Typically, gem cutters aim for a well balanced color split and show it off with a rectangular step cut.
I am unsure of the particulars of how the coloring occurs naturally, but this is how it looks:
This distinctive color zoning, in the hands of an imaginative gem cutter, can yield striking stones.
Ametrine can be produced through careful heating of amethyst, as well. Heat changes the purple to yellow.
In 1994, a Russian lab perfected the synthesis of bi-color quartz, so beware of high quality goods at low prices or sold as Russian in origin.
This is a beautiful example of synthetic quartz. Note the seed plate in the center:
And to end on a happy note, I leave you with a particularly artful example of unconventional lapidary: