Prasiolite is often referred to by the trade name of green amethyst, and that is what most people would recognise when presented with this delicate green stone. It's actually incorrect and in fact, we're not supposed to refer to it as such; amethyst is the purple variety of quartz, and so by definition you can't have green amethyst! It's like referring to red beryl as 'red emerald' or heliodor as 'yellow emerald'*: it just doesn't make sense. So what we have here is the green form of quartz. This can form naturally, but such stones are very rare and so most of what you will find on the market is either amethyst or citrine that has been either heated, irrradiated or both. It can only be produced from mines in certain locations, and Brazil is still the main source for this stone. It's a lovely, delicate colour and it's not hard to find clean quality stones in quite large sizes at a reasonable price.
*examples provided by gemology.com