There is a version of amethyst's story that gets told in crystal shops, and it involves Greek wine and a nymph and a god with a temper. It is a good story. It is also a Renaissance invention — the tale first appears in a French poem of 1576, not in any classical source — and the real history is more interesting, because it tells you why the stone on your shelf costs eight pounds instead of eight hundred.
One of the five
Until the nineteenth century, amethyst was a cardinal gem: one of five stones — with diamond, ruby, emerald and sapphire — considered precious, with everything else lumped in as semi-precious. It was a stone of bishops and monarchs. Amethysts sit in the British Crown Jewels. Catherine the Great collected them in quantity. The papal Ring of the Fisherman is traditionally set with one.
Then, in the 1830s and 1840s, a German immigrant to southern Brazil — a man from the Idar-Oberstein gem-cutting district, who therefore knew exactly what he was looking at — found an enormous deposit. Others from the same region followed and began shipping rough back to Germany to be cut. Within a few decades the supply had multiplied beyond anything the market had seen, and amethyst fell out of the precious category for good.
Nothing about the stone changed. Only the quantity of it did. That is worth sitting with, because it is the clearest illustration you will find that a gemstone's price is a fact about geology and shipping, not about the stone's nature.
The colour is damage, and it is reversible
Amethyst is quartz. Chemically it is silicon dioxide, identical to the clear stuff. The purple comes from traces of iron in the crystal lattice that have been altered by natural radiation over geological time — a colour centre, in the trade's language, and a fragile one.
Heat it and the colour changes. This is not obscure: the great majority of citrine sold is heated amethyst or heated smoky quartz, treated at or near the mine, and the trade accepts the practice as standard because the result is stable. Natural citrine exists and is comparatively scarce. If you have bought a bright orange-red citrine at a friendly price, it was almost certainly purple last year.
Which brings us to the sunlight problem
A great deal of crystal-care advice tells you to charge your stones on a sunny windowsill. For amethyst, this is the one piece of advice that will actually damage it. The same colour centres that heat destroys are also degraded by ultraviolet light. Leave a good amethyst in direct sun for a summer and it will come out paler than it went in. Rose quartz fades the same way, and for the same reason.
Moonlight, whatever else you think of it, is at least harmless.
What to look at when you are choosing one
Colour distribution is the honest test. Amethyst almost never colours evenly — it grows in zones, and in a cut stone a cutter's skill is largely a matter of orienting those zones so the face-up colour looks uniform. In a raw cluster you can see the banding directly: the tips are usually the deepest purple, the base often nearly white. That gradient is a signature of the real thing.
Very even, very saturated purple in a cheap tumbled stone is worth a second look. Dyed quartz exists, and dye pools in the fissures.
Our own amethyst clusters are sold by weight range rather than a fixed size, because grading natural specimens more finely than that would be a fiction.