What is streak? Why is it a useful diagnostic techique in the identification of minerals?

Question

What are the limitations of this technique?

Answer 1

Streak is the fine powder given off by the mineral when rubbed across a porcelain plate (non-glazed).  Many minerals have a different color when powdered than they do in crystal or massive forms. The color may be entirely different, or it may be a different shade.  And from this streak color, geologists can determine the mineral.  Limitations of this technique is if the mineral tested is harder than about 6 on the Mohs hardness scale; all it will do is scratch the streak plate and not leave a powder streak.