What is streak?
The streak of a mineral is the color of the powder left on a streak plate (piece of unglazed porcelain) when the mineral is scraped across it. The streak plate has a hardness of glass, so minerals with a Mohs Hardness >7 will scratch the streak plate and won't powder the mineral.

Where do you observe streak?
A mineral’s streak is determined by rubbing it on a streak plate, which is a piece of unglazed porcelain. The streak plate is essentially a type of glass, so it isn't used on minerals with hardness greater than 7.

How useful is streak in identifying minerals?
Streak can be useful for identifying metallic and earthy minerals. Nonmetallic minerals usually give a white streak because they are very light-colored. Other minerals may have very distinctive streaks. Hematite, for example, always gives a reddish-brown streak no matter what type of luster it displays.