Obsidian is a black rock left behind in the aftermath of a volcanic eruption. It is a fairly brittle metal but extremely sharp, making it fairly useful for improvised tools in regions where volcanic activity is high.
Being that obsidian is a side effect of volcanic destruction, which very often kills every living thing in its path, it holds a great importance to worshipers of Artemis. The black rock symbolizes the cycle of death and eventual rebirth, and is used as a catalyst in rituals. Its sharpness also makes it ideal for sacrificial rites of the flesh.