Amethyst and citrine: same mineral, different color, and a real trade secret

Amethyst and citrine are both quartz colored by trace iron, the difference comes down to the oxidation state of that iron and, in many cases, heat. A genuinely useful fact for collectors: most commercial citrine on the market is actually heat treated amethyst, since natural citrine is comparatively rare and heating amethyst to a few hundred degrees reliably shifts it toward yellow or orange. This is a long standing, well known practice in the gem trade, not a scam, but it does mean color alone cannot tell you whether a piece is natural citrine or heat treated amethyst. Amethyst can also fade with prolonged sun exposure, a real and sometimes surprising property for a stone that is otherwise quite durable at 7 on the Mohs scale.