Streak is a term used to describe the color of a mineral's powder when rubbed against a streak plate (unglazed porcelain). Obsidian is a volcanic glass, not a mineral, and it's too hard to leave a powder on a streak plate.

Instead of a streak, obsidian is identified by its shiny, glassy fracture, its dark color (often black, but can be brown, green, or red), and its conchoidal fracture (which creates smooth, curved surfaces).