Obsidian is formed by rapidly cooling molten rock or lava. Obsidian is also called volcanic glass, and it is considered a mineraloid.
Examples of the rapid cooling that forms obsidian are lava oozing into a body of water, or lava splashing against something and bouncing into the air. Most obsidian is black; however, it can also be found in other colors. Scientists believe the colors come from other elements mixing in with the molten rock just before the rapid cooling occurs.
Most obsidian found is from volcanic eruptions that occurred within the past few million years. That is because older obsidian has most likely shattered into dust over time, as stated by geology.com.