Citrine

Here is something I feel strongly about being upfront on: the vast majority of citrine sold commercially — including most of the deeply saturated, rich-orange pieces you will see labelled ‘Madeira citrine’ or ‘Brazilian citrine’ — is heat-treated amethyst. When amethyst is heated to between 400 and 500°C, the irradiation-based purple colour centres are destroyed and the iron impurities produce yellow-orange tones instead. The resulting stone is mineralogically identical to natural citrine (same formula, same crystal system, same hardness), but the colour is the product of a kiln rather than geological time. This is not fraud in itself — the treatment is stable, widely practised, and the material is still real quartz — but it is worth knowing, and responsible sellers should disclose it. You can often identify treated material by its deeply saturated orange-red tones and by the white, almost milky base at the bottom of a crystal cluster, where the heat affected the less-iron-rich quartz differently.

Natural citrine is a genuinely rarer stone. Spanish citrine from the Salamanca region is among the most historically significant, a pale golden-yellow material that has been mined since Roman times. Natural Brazilian material tends toward softer, smoky-yellow tones rather than the vivid orange of the heat-treated variety. From a geological standpoint, the colour difference between natural citrine and amethyst is primarily about the oxidation state of the iron impurities — Fe²⁺ versus Fe³⁺, and the presence or absence of the irradiation that creates the purple charge-transfer centres. It is a small chemical difference with a large visual consequence, which is something I find genuinely interesting: the entire colour identity of two of the world’s most popular gemstones comes down to the oxidation state of trace quantities of a single element.