Black glass. If it has white bits in, it is called snowflake obsidian. The piece on the right shows a conchoidal fracture.
Some minerals break easily into flat surfaces, like calcite. These are called cleavage planes. Other minerals break open with an irregular surface. This is a fracture. A conchoidal fracture has curved lines, rather like a sea-shell (which is where the word "conchoidal" comes from). Obsidian has conchoidal fractures, as does ordinary glass.
Obsidian is volcanic glass. It is an igneous rock, which means that it has been made through fire (that is, volcanoes).
We tend to think of stone-age tools as flint. Yet many cultures used obsidian instead. You can work obsidian to a sharp edge. You can see that the specimen on the left has a fairly sharp edge at the bottom naturally. The Aztecs cut open the bodies of their sacrificial victims with obsidian knives. Obsidian was also used and traded in the Middle East in prehistoric times.
Larger pictures of Obsidian:
The first two photographs show both sides of the same specimen. One side shows the broken, shiny surface, and the other, the pitted outside, with other volcanic minerals embedded in it.
Obsidian is sometimes patterned. This is Mahogany Obsidian.
The next three are Snowflake Obsidian. The first is unpolished, and the next two, polished.
The next two are polished plain obsidian.
This natural specimen is of a form called an Apache Tear.
This specimen is transluscent. You can see the light shining through it.
Obsidian used to be used to make stone tools. This is a modern replica.
