Obsidian is a volcanic glass, so it is not technically a crystal, but it has properties like crystals do. Each type of obsidian has different properties which are linked below.
Obsidian forms when there is a lot of silica present and lava cools rapidly. It is around a 5 on the Mohs scale of hardness, and it can form sharp edges that were used as arrowheads and scrapers.
Pure obsidian is black, but impurities create stones like snowflake obsidian, golden obsidian, and others.