17.8g, 3.5cm. Obsidian, also known as volcanic glass, is an igneous rock that is formed during the rapid cooling of acidic lava (e.g. in water) so that individual minerals do not crystallize. It is basically a natural form of silica glass. Silver obsidian has a gray-black color and a distinct silvery luster, which is caused by the presence of small air bubbles and impurities inside the obsidian. The most beautiful silver obsidians come from Armenia from the area around the city of Sevan.