Very often referred to as "volcanic glass" Obsidian is neither a rock or a mineral. It is classed as a mineraloid because it is not crystalline. Odsidian is formed in rhyolitic eruptions, and cools so quickly that crystals do not have time to form. Its composition is generally 70% of SiO2 The three specimens are from Mexico, The larger one is colloquially known as Snowflake obsidian. The snowflake appearance is due to inclusions of spherulites in the form of clusters of radiating crystals of cristobalite. The brown specimen is coloured by trace amounts of iron.