Obsidian is a volcanic glass that cools from the molten lava that is spewed out from volcanoes. Apart from its black color , it behaves very similarly to the window glass. In fact obsidian is mostly composed of silicon dioxide (SiO₂), which also happens to be the composition of glass. The reason obsidian appears black instead of transparent like glass is due to the presence of other minerals, like iron and magnesium.

Obsidian can be found anywhere there are rhyolitic volcanoes (a type of volcano where there a lot of silicon dioxide erupts, which causes the lava to be more thick and viscous).

Obsidian is a part of a range of rocks that can form from a volcano eruption; other types of rocks can be found here . Historically, obsidian has been used to make tools, mirrors, and decorations. If you are interested, you can learn more about obsidian (and the study of other minerals and rock in geology) here .

Obsidian is a volcanic rock that is not made of crystals, i.e. it is a glass. It is similar to glass you are familiar with, but has impurities in it that make it dark in color.