This gemstone was cut by my husband, Mike, a talented faceter! It is a Prasiolite, which is a treated quartz. There are three ways to create this color, one is by irradiation, one is by heat treatment, and the third is by a natural heat treatment which is while the stone is still in the ground and is due to volcanism.
Apparently you start with a natural amethyst before the treatment begins. Even though you start with a natural amethyst, the term "green amethyst" is actually a misnomer and should not be used to describe the stone commercially. It has not been an easy stone to find in jewelry stores because it is not widely known by the public yet.
The photographs appear to show a slight yellow cast in the stone. The color is actually the pale mint green color that is so beautiful. This stone weighs approximately 11.5 carats, measures 14 x 14 mm, and it is cut into a pattern by the name "Lydia's Barion", and is a real sparkler!