The definition of streak is a powder left when you scrape a mineral across a porcelain tile. If you rub a rock against a tile, it will make a mark. That mark is called a streak. Streaks can be different colors than the original rock. Some rocks can even have no streak. This is an example of two forms of hematite, one with a metallic luster and one with an earthy luster, but both leave a brownish streak. Chalcanthite is a blue mineral, but leaves a white streak.