The streak of a mineral is the color of its powder.
This powder varies in color, and may be white, gray, red, etc. It is
obtained by scratching the mineral with a sharp file, or by rubbing the
mineral on the back of an unglazed porcelain plate, when the color of
the powder will appear on the plate.
It is remarkable that the streak of the diamond is gray to
grayish-black, while that of the ruby is colorless or white.