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Now we're going to find out more about the streak test. Rub a sample of pyrite or chalcopyrite ("chall-co-PIE-rite")across a piece of unglazed porcelain tile.
The greenish-black streak is from the mineral in the rock. The color of the streak made by mineral is often different from color of the mineral in the rock.
Rub other rocks against the tile. Use the Color Sorting Key for Rocks, which lists the external coloring and the mineral streak made by some common minerals, to identify the minerals in the rocks.
Compare the color differences between the rock colors and the streaks made by minerals.
(This writer has come to believe that brilliant geologists learn what kinds of minerals make which streak-colors, so they can tell what kind of mineral even though the streak - color is not the same as the color of the mineral in the rock.)
You will begin to understand that a color streak is useful in the identification of some minerals but that a white streak is not.
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