Fused Silicon Dioxide (SiO2), AKA silica glass. If you find a piece of glass formed naturally, you have a chunk of lechatelierite.

This is a mineraloid that is often classed in the quartz group, although lechatelierites do not have a crystalline structure (they are fused at high temperatures). Lechatelierite does not include obsidian, which has large amounts of impurities; lechatelierites are pure or nearly pure. They are extremely rare, and all the more so because glass is so breakable. While granite may stick around for millions of years, glass doesn't tend to weather well in a geological time frame.

There are three potential natural causes of lechatelierites; meteorite impacts, volcanism, and lightning strikes. Lechatelierite caused by a lightning strike is called fulgurite; to the best of my knowledge there aren't any special names for the other two types, although the term tektite is used somewhat informally to refer to samples that may have been formed by meteorites. Lechatelierite has also been created in laboratory conditions, by putting a strong electrical charge through sand.