The material is known as Mount St. Helens glass, which is a man-made gem material that is made from the welded ash tuff that was erupted out of that volcano during the 1980 explosion. The green was the first color that was made and its color rivals that of a nice emerald.
This piece is unusual in that it looks like a crystal, rather than a faceted gem. It was likely faceted by someone to resemble a crystal, since glass is not by nature crystalline in any way or shape...but it's still pretty.
One of my first jobs was working for a geologist with the US Geological Survey who was one of the many scientists who measured and watched the mountain as it puffed up, reared back, and belched that massive cloud of ash back then. It was exciting to hear him talk in depth about the place - even though what I did was to color his maps. So every time I look at this, I smile, think of all the measurements and photos (and broken pencils), and reminisce.
------------------------- Mount St. Helens glass (also called "helenite"); sterling silver square and half-round wire
It is such a gorgeous color, and looks nice in this shape and wrap style. I alway like those staggered elements on the sides. It's like a magic crystal out of a fantasy, especially wrapped like this