Obsidian was probably the Earth's first glass.
Obsidian is volcanic glass and is usually black in color though brown and green obsidian also exists. It is formed when lava with a high percentage of feldspar (a silica mineral) cools too quickly for crystals to form.
Obsidian is slightly harder than window (float) glass and has been used throughout history for its functional properties. An obsidian shard can be sharper than surgical steel. For artists it has been both a medium and tool.