Minerals with a hardness above 7 will not leave a streak because they are harder than the porcelain. The streak test is useful in distinguishing between native metals and metal ores, both of which show metallic luster. Streak is useful in identifying minerals that occur in many colors. For example, quartz may be found in almost any color, but its streak is always white. Cleavage is the splitting of a mineral in definite directions because of the internal structure of the crystals present.
Most minerals break along a certain plane.
Mica cleaves into thin sheets. Galena breaks into little cubes. Minerals also break without cleavage or across cleavage planes. These characteristic fractures are also helpful in mineral identification. Quartz and malachite fracture with shell-like curves.