Ametrine forms when part of an amethyst crystal is heated naturally by the ground around it, turning that section gold while the rest stays purple; the boundary between the two is often startlingly sharp. Nearly all natural ametrine comes from the Anahi mine in Bolivia. It is quartz, 7 on the Mohs scale, hard-wearing and safe in water, though strong sunlight will pale the purple over time. Heat-treated imitations exist, so a clean natural boundary is worth looking for. Hand-picked in St Andrews and posted UK-wide.