Obsidian as natural glass

Natural amorphous oxide stone, usually black. Used to anchor the definition of glass as a non-crystalline solid in the context of ancient glass-making from slags.

Because we'd like something transparent to keep the weather out, keep the wind out, and let the light in, we've had glasses since about 3500 BC. The first glasses were in Syria, and it was probably the result of refining metals — they had slags, and the slags in many cases were molten oxide, and they were glassy. Initially they took these molten slags and probably made some little shape out of it, and eventually they started making art objects out of glass. They were very expensive, and the glass wasn't controlled composition. A lot of it was black. Does anybody know the name of the stone that is basically a form of glass? It's called obsidian. Usually black. It's basically a mixture of molten oxide, and it's not very useful for transmitting light, but it has an amorphous structure. A glass essentially has no crystalline structure if you want to define it in modern physics terms.