Can amethyst be heated to produce citrine?

Yes, the vast majority of commercial citrine is made by heating pale or lower-grade amethyst, usually somewhere around 470°C up to 750°C. That controlled thermal step changes the colour permanently, so the stone goes from purple to yellow, or sometimes to deep orange.

This change happens because the strong heat shifts the oxidation state of tiny iron impurities sitting inside the quartz crystal lattice. Even though natural citrine is rather uncommon in the wild, heat-processed amethyst is plentiful, structurally steady, and well accepted across the commercial jewellery industry. In many cases, gemmologists can spot the heated origin from a faint reddish tone, and also the absence of dichroism, an optical effect that is commonly seen in natural citrine.