"Fire glass produces more heat than real wood, and is also environmentally friendly. There is no smoke, it's odorless and doesn't produce ash. You are able to stay toasty warm without cutting down trees and the specially formulated glass crystals give off no toxic deposit.."
I actually stumbled across this on a buzzfeed article about pinterest (I will just come out and say it, if you want bad pinterest ideas, buzzfeed is a hotbed of them) and knew right away that couldn't be right. I searched pinterest curious to see how often this is actually pinned, and was floored that the above image has been repinned nearly 5,000 times from one pin alone.
A quick Google search later and sure enough, I found the website for Fire Glass. Pulled directly from their FAQs:
"Fire Glass is tempered glass that is used in fireplaces and fire pits to increase vibrancy, reflection and color. Tempered glass is tumbled and polished to prevent sharp edges and injury. The glass is designed for gas fire pits and gas fireplaces to tolerate high temperatures without melting, burning or discoloring when used as recommended. The glass does NOT create toxic fumes, smoke, ash, or soot... You can use fire glass with both propane and natural gas. "
What does all that mean? It's not a "flammable glass". It's a filler used in gas or propane fireplaces and fire pits; instead of lava rocks or fake wood, you add Fire Glass. The glass itself isn't flammable and won't do anything if you try to set it on fire. It's not a replacement for real wood in a wood burning fire pit. It is quite pretty and I can see the appeal, but unless you have a gas burning pit, it's not going to do anything.