Prasiolite is light mint green quartz, the brother of amethyst and citrine. It is also sometimes incorrectly referred to as “Green Amethyst” . Prasiolite is the result of an amethyst (purple quartz) gemstone being exposed to high heat. This process can occur naturally inside the earth, or by humans in a laboratory. Not every amethyst will produce prasiolite when heated (some will turn into citrine). Prasiolite that is not treated is very rare and most of it comes from one mine in Brazil.
Prasiolite, like almost all quartz, can be found in very large sizes. Its range of clarity is similar to that of amethyst and very clear stones are not hard to find. The most important factor in determining the value of a piece of prasiolite is its color – the richer, more saturated green, the better. However, by its nature, prasiolite will never be dark. In fact, a lot of lab-grown quartz is sold unscrupulously (e.g. on ebay) as prasiolite. If it is very dark or rich in color, stay away.