from JTV's Gemopedia (TM)::
     A pastel dream, this variety of quartz boasts a beautiful soft green color. Sometimes called "green amethyst" or "lime citrine," prasiolite is rarely found in nature, so most material available on the market is produced by heating or irradiating amethyst or citrine. Prasiolite is an exciting gem in that it is readily available in large sizes with high transparency and great durability.

from the internet:

     Prasiolite (or vermarine) is the name for any quartz crystal or cluster that is green in color. Although often cut as a gemstone, prasiolite is actually quite rare in nature.

      Most prasiolites on the market are heat treated amethyst These stones are sometimes called "Green Amethyst", possibly because amethyst is the starting variety, or possibly just because it sounds more valuable that way. When heat treated, the amethyst from most locations transforms into yellow or orange citrine. From a few locations, a green color results. These locations include Montezuma, Brazil, and Arizona. Naturally occurring prasiolite has been reported from Poland.