The rubbing of an unidentified mineral against an abrasive tile (usually ceramic) for the purpose of examining the color of the small grains scraped off. The streak test is useful as a diagnostic in many cases because, in naturally occurring crystals, the large particle size makes them opaque, producing a dark color. Crushing the particles into a smaller size allows their "true" colors to be seen. The technique does not work, however, for some extremely fine-grain minerals such as magnetite.