Igneous rocks may be classified by their overall color, which is a clue to their chemical and mineral composition. Geologists call most dark-colored igneous rocks, such as gabbro and basalt, mafic (pronounced "MAY-fic") rocks. In the word "mafic," the first syllable represents the element "magnesium" and the F stands for "ferric," an adjective describing the element iron. Mafic rocks have a relatively high content of iron, magnesium, and calcium and less silicon dioxide than other types of igneous rocks. They also begin to solidify at higher temperatures, usually above 1,000 degrees C (1,800 degrees F). By contrast, felsic rocks, such as granite and rhyolite, are lighter in color overall, though they may contain dark grains. "Felsic" is a word coined from the minerals "feldspar" and "silica" (quartz); felsic rocks contain 70 to 75 percent silica and do not start to crystallize until magma, or molten rock, cools to around 700 degrees C (1,300 degrees F). In other words, in a body of magma that is cooling down, the mafic minerals will tend to form before most of the felsic minerals.