A large chunk of obsidian, between 2" and 4" in size. Be careful handling this, as the edges can be quite sharp! The piece of obsidian you receive will vary, but all are very dark. Sometimes a redder or browner stone will be attached.
Obsidian is a kind of natural glass formed from lava. It forms with rapid cooling, too quickly to form any crystal structures. This stone is hard but brittle, and its incredibly sharp edges have historically made it useful as a blade or sharp tool as far back as the neolithic era, and there are even modern surgical tools made from obsidian!
As is the case for most black stones, obsidian is a favourite to be used as a protection stone. Its ability to take sharp edges makes it even better than most other black stones for protective work. For the same reason, it's a helpful stone for use in cursework.
When cut into flat planes and polished, black obsidian forms a reflective "mirror." These and other large black obsidian pieces have been used for Scrying. Because of this connection to scrying, some people use it for meditation or for divination of other kinds.
Obsidian is a very truthful stone. For scrying, protection, or really any use you put it to, it will cut through to the core of any problem, and be merciless in exposing weakness and flaws. This can be very helpful, but also can be very intense. For a gentler form of obsidian to work with - snowflake or "Apache tears" can be easier. Snowflake obsidian is wonderful for protection work with children.