Obsidian. Already used in the Stone Age as ornament, and as a tool; highly regarded due to its cutting ability. Nowadays obsidian scalpels are still used in heart surgery, its edge is very thin, so it causes less damage, allowing surgical wounds to heal quickly.
Obsidian is black, although it may change depending on the presence of impurities and the way it is cut. If it is cut in parallel it has a black colour, if cut in perpendicular it is gray. As a result of these colours, we can find different kinds of obsidian:
Snowflake obsidian, Mahogany obsidian, Rainbow obsidian.
Obsidian is known as the stone of justice, it has properties that favour meditation. It is suited for treatment against depressions, anxiety and nervousness.