The variety of gemstone that is formed from natural volcanic glass is called "obsidian." Obsidian is a naturally occurring volcanic glass that forms when molten lava cools rapidly and does not have enough time to crystallize. It has a smooth, glassy texture and can come in various colors, including black, brown, gray, and even iridescent hues. While obsidian is not a traditional crystalline mineral like many other gemstones, it is still considered a valuable and attractive material for use in jewelry and decorative objects due to its unique appearance.