Obsidian is a common black low-grade gem, also known as natural glass, which is a naturally occurring glass. It is caused by the rapid cooling and condensation of volcanic lava and its crystal structure. Obsidian usually appears on the periphery of lava flows because it cools fastest. Obsidian is usually black, but brown, gray and a small amount of red, blue and even green substances can also be seen. Because the microcrystals, chemical inclusions and shape inclusions in obsidian are all arranged in one direction, it presents rainbow effect under the interference of visible light, including 30-sided rainbow effect and double-sided rainbow effect. This is a very precious wealth.