Often the colour of a mineral is different when it is in a single lump or when it is powdered.
The colour of the powdered form is called the streak.
The most common method of getting this powdered mineral is to rub the sample on the unglazed surface of a ceramic tile.
Sometimes the mineral is harder than the tile.
(See Hardness)
Crushing the mineral and rubbing it onto a white surface will reveal the streak.