I think there was a thread that talked about this about a year ago, because every time I see "green amethyst" I think "well, kinda..." If I remember correctly, green amethyst is prasiolite and it does have to do with heat treatment.
Yes, it''s prasiolite. Natural prasiolite is very rare but it occurs. It''s been known for many years that heat, or more recently a combination of irradiation and heat, will change amethyst from some locations into a shade of green. It used to be marketed as "greened amethyst," implying the green color was not natural. Richard M.