Obsidian rock is a natural
It is an extrusive
which formed at so rapid cooling of viscous, felsic lava that
mineral crystals didn't manage to form.
Its main component is glass,
often, no minerals
Occasionally, though, rare quartz and feldspar phenocrysts
can be found.
Those have formed underground, before the lava had
erupted and obsidian was formed. It also contains tiny amounts of
In fact, like with pegmatite
the term "obsidian" only refers to its glassy texture.
Theoretically, a rock with any composition
can be it. In practice, though, it most often has the composition
similar to rhyolite.
Like rhyolite, it can have a flow-banding texture, and it has got a
sharp, concoidal fracture.
This is why it has been used to make cutting
tools since ancient times.
It is most often black, but can occasionally
be reddish or have a white snowflake pattern.