Some types of obsidian include snowflake obsidian, rainbow obsidian, black obsidian, mahogany obsidian and golden sheen obsidian. Obsidian is an amorphous, non-crystalline glass composed primarily of silicon dioxide. Types of obsidian are differentiated by variations in their appearance that generally result from the presence of impurities and how the impurities are distributed.
Pure obsidian is typically black, but contaminants can change its color. The presence of iron or magnesium tend to turn it dark-brown, while traces of other elements can turn it blue, yellow, orange or red. Sometimes contaminants are arranged in streaks, causing rays or swirls of color in some types, such as in rainbow obsidian. In mahogany obsidian, pure obsidian and iron- or magnesium-contaminated brown obsidian swirl together in a single specimen. Inclusions of a white mineral called cristobalite sometimes form radial clusters in obsidian to form snowflake obsidian. In sheen obsidians, such as golden sheen and mahogany sheen, the presence of layers of tiny gas bubbles produce a shiny appearance, or "sheen."
Obsidian is an igneous rock that typically forms on the Earth's surface. It forms when molten rock cools so rapidly that atoms do not arrange in a crystalline structure, such as when lava flows into water. Obsidian is not a mineral and is sometimes called a "mineraloid" because of its mineral-like appearance.