Aren't they great! It's Called Fyre Glass (Fire Glass). It comes in different sizes, colors and even has 'mirrored' pieces....take a look at this link: http://www.rhpeterson.com/realfyre/vented-series/contemporary-series/fyre-gems/. This will give you an idea of what is out there. Soo fun to work with! It's a little pricey though....but it lasts for a long time!

These are some type of "glass ember" as opposed to stone.

they are like tempered glass i believe. They don't melt or change form. Some do get ash on them over time, so you just have to watch out for that.