Obsidian rock. Polished slab of obsidian rock. Obsidian is an igneous rock formed by the rapid cooling of volcanic magma or lava which contains granite. Obsidian has a glass-like structure because it contains high levels of silica, the basic component of glass. The rock's black colour is due to the presence of small amounts of iron. When obsidian rock is speckled with white dots of cristobalite (as shown here) the rock may be called 'snowflake obsidian'. This particular sample is 240mm across.