What is the streak for diamonds?
Diamond does not streak. Anything with a Mohs hardness greater than about 7 will cut the ceramic plate used for the test. Diamond, the hardest naturally occurring substance, has a Mohs hardness of 10. (Nothing else in nature comes close.)
Diamond will not leave a streak on a streak plate because diamond is harder than the unglazed porcelain streak plate. The streak is the color of the powdered mineral residue left on the streak plate surface when pulling a mineral across it. With a Mohs hardness of 10 (on a scale of 1-10) a streak could only be left on a diamond plated streak plate. Diamond streak is colorless.