Color is the easiest of the properties to see, but it is not always the best way to identify a mineral. Many minerals have more than one color because of impurities that were present during the formation of the mineral. Quartz is an example of a mineral with many different colors. Quartz can be clear, white, blue, brown, and almost black. Amethyst is a quartz crystal with a purple color. The impurity that makes amethyst purple is manganese.

A better determinant of the true color of a mineral is its streak. Streak is a test used by a geologist to see the color of the mineral under the top layer or coating on the mineral. The mineral is rubbed on a "streak plate", which is a piece of porcelain. When the mineral is rubbed across the streak plate some of the mineral is broken off and ground into a powder. This allows the geologist to see under the outer layer which could have a different color due to the mineral being exposed to the atmosphere. When minerals are exposed to the atmosphere, gasses like oxygen can chemically combine with the mineral to change its outer color.

The photo above is showing a specimen of iron pyrite along with a streak plate showing the pyrite's streak.

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