Think warm sunshine trapped in a stone — that’s citrine. It’s the yellow-to-orange variety of quartz, beloved for its sunny palette and toughness (Mohs ~7), and is November’s second gemstone. But when you start digging into where it comes from, the story gets a little twisty: true natural citrine is uncommon, most commercial “citrine” is heat-treated amethyst or smoky quartz, and a very special sibling — ametrine — is essentially amethyst and citrine in one crystal and comes from only one commercial source. Let's walk through where citrine is found, trade names and shades, how heat treatment turns amethyst into citrine (with the geology explained plainly) and amazing museum specimens.…