Colour of a mineral in its powdered form, usually obtained by rubbing the mineral on a hard, white surface (e.g., a tile of unglazed porcelain) to yield a streak of fine powder. The streak's colour is usually constant for a given mineral, even if the mineral varies in colour as it occurs in the field or if the streak is different from the colour of the unpowdered mineral. Streak is diagnostically useful because it may distinguish between mineral species that are otherwise similar in appearance.
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