Specimens of green obsidian from the U.S.A. These samples of obsidian have formed with a dark green colour due to the presence of magnesium and iron during their formation. Obsidian is a natural volcanic glass, an extrusive igneous rock, it is produced when silica rich lava extruded from a volcano rapidly cools minimising crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows. The high silica content (70 -75%) produces a highly viscous lava with some degree of polymerization. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. It is classed as metastable as over time it will crystalise. Non has been found over Cretaceous in age. Obsidian is hard and brittle with a concoidal fracture it therefore breaks with very sharp edges, which were used in the past in cutting and piercing tools. It has been used experimentally as scalpel blades. These samples are supplied in a 2" x 2" white card tray with an information label, the samples are a must have for any rock collection due to their unusual and somewhat rare colouration.