Ametrines

Ametrine is a single quartz crystal that grew as two colors at once — purple amethyst on one side, golden citrine on the other, meeting along a boundary that can be knife-sharp or softly blended. It is one of the few gems where the interesting thing is a line rather than a color.

How that boundary reads is the cutter's decision, which is why so many ametrines are emerald cuts: a long open step cut runs the split down the length of the stone and shows both zones at once. Mohs 7, like all quartz. Standard millimeter sizes, and these run large.