Obsidian is an igneous rock that forms when molten rock material cools so quickly that atoms are unable to form themselves into a crystalline structure. The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture.
Obsidian is usually an extrusive rock. One that solidifies above Earth's surface. However, it can form in a variety of cooling environments:
Along the edges of a lava flow (extrusive)
Around the edges of a sill or a dike (intrusive)
Where lava contacs water (extrusive)
Where lava cools while airborne (extrusive)
Black is the most common color of obsidian. However, it can also be brown or green. Sometimes, obsidian can be blue, red, orange or yellow.
Sometimes, two colors of obsidian will be swirled together in a single specimen. The most common color combination is black and brown obsidian swirled together, that's called "mahogany obsidian"
As a "glass", obsidian is chemically unstable. With the time, obsidian starts to crystallize. This process does not happen at a uniform rate throughout the rock. Instead it begins at various locations within the rock. At these locations the crystallization process forms radial clusters of white or gray cristobalite crystals within the obsidian. When cut and polished these specimens are referred to as "snowflake obsidian"