Ametrine is the trade name for bi-colored amethyst; a stone which contains both amethyst and citrine in the same crystal. It's relatively scarce, as citrine is amethyst that has been heated, so to find them both in the same crystal means that that crystal has been heated in some parts but not others. There is only one mine that produces ametrine, which is in Bolivia, close to the Brazilian border. It's often cut as a rectangular step cut, to display the bi-colour effect, but it can produce some spectacular effects in brilliant or fantasy cuts. I have just a few rectangular step cuts and some rectangular concave cuts.