Back to Materials Obsidian Obsidian is a volcanic glass that is formed when felsic lava (rich in light elements like silicon, oxygen and aluminium) cools too quickly for crystal growth to take place. Obviously found around many volcano`s around the world, it is a common material, and has been used widely over many centuries as a cutting edge. Because it is a glass rather than a crystal sharper edges can be worked onto the substance. Obsidian has a low water content around 1% by weight. Over a long period of time obsidian will become more and more hydrated and eventually turns into perlite, however not in the lifetime of meer mortals. We use a type of obsidian called snowflake obsidian. There are gray/white patches on the obsidian which are crystals of cristobalite. Obsidian has a specific gravity of approximately 2.50 (2.50 times the weight of water by volume), and its hardness value is 5.0-5.5 on the mohs scale.