The color index of igneous rocks is a very useful indicator of the types of minerals present in the rock and therefore the specific type of rock. The color index of an igneous rock is a measure of the ratio of dark colored, or mafic, minerals to light colored, or felsic, minerals.
According to Bowen's Reaction series, minerals crystallize at different temperatures depending on their chemical composition. At high temperatures only minerals that have structures stable under those conditions will be able to crystallize. Typically those minerals are olivine, pyroxene, and plagioclase feldspar. Therefore igneous rock that forms at high temperature would be composed of these minerals and hence be dark or mafic in color.
Rocks that have formed from magma at relatively low temperatures would be composed of the lighter colored quartzes, potassium feldspar, and muscovite mica. These rocks are termed felsic because of their typically high content of both feldspar and silica.
To begin identifying your igneous rock determine its color index based on the scale below and click on the appropriate link in the menu on the left