Obsidian is an extreme variety of igneous rock with a glassy texture. Most popular accounts say that obsidian forms when lava cools very quickly, but that is not quite accurate. Obsidian starts with a lava very high in silica (more than about 70 percent), such as a rhyolite. The many strong chemical bonds between silicon and oxygen make such a lava very viscous, but equally important is that the temperature range between fully liquid (the liquidus) and fully solid (the solidus) is very small. Thus obsidian does not need to cool especially fast, because it solidifies especially fast.
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