Is a piece of obsidian a pure substance?

Obsidian is not considered a pure substance; rather, it is a naturally occurring volcanic glass that consists primarily of silica (SiO2) along with various trace elements and minerals. While it has a relatively uniform composition, the presence of these impurities and variations in its formation process means it does not meet the strict definition of a pure substance, which is typically a material with a consistent and uniform composition throughout.