Citrine is yellow to orange quartz, and the single most important fact about the commercial stone is that most of it did not start out that way.
Natural citrine, coloured by traces of iron in the quartz structure, is genuinely uncommon and is usually a pale, slightly smoky yellow. Amethyst, coloured by iron in a different oxidation state produced by natural irradiation, is abundant. Heating amethyst to somewhere in the region of 400 to 500 degrees Celsius converts the colour centres and turns the stone yellow, orange or reddish brown, permanently and irreversibly. The process is cheap, it is carried out on an industrial scale on Brazilian and Uruguayan amethyst, and the resulting material fills the world's citrine counters.
Some indicators distinguish it. Heated amethyst often shows a strong orange to reddish cast and colour zoning inherited from the original crystal, and the trade name Madeira citrine generally denotes it. Natural citrine tends to be paler and more evenly coloured. Neither test is conclusive without laboratory work, and disclosure practice in the trade is inconsistent.
The confusion runs deeper than treatment. Citrine and yellow topaz were not distinguished in the older trade at all, and citrine was sold as topaz for centuries. That is why November carries both stones in the modern birthstone lists, and why any pre-twentieth-century reference to a yellow topaz has to be treated as an unresolved identification.
None of the traditional attributions attached to citrine can be assumed to describe the stone now sold under the name.