Final answer:
Colors in igneous rocks like dunite aid in classification by indicating mineral content, such as olivine determining a dark green color for ultramafic rocks. Metamorphic rocks show mineral content through banding and texture. Colors from mineral oxides are also used in natural pigments.
Step-by-step explanation:
Identifying minerals in an igneous rock such as dunite, which is characterized by a dark green to black color due to a high content of olivine, is crucial for geological classification. The color of a rock can be indicative of its mineral content: ultramafic rocks are usually dark green because of olivine, mafic rocks are dark-colored as they contain ferromagnesian minerals, and felsic rocks, with a large amount of nonferromagnesian minerals, are light in color.
When grains are too small to see, as in some extrusive igneous rocks, geologists refer to the overall color and texture. For instance, they might deduce dark colors from iron and magnesium-rich minerals, such as olivine and pyroxene, while light colors could indicate the presence of minerals like quartz and potassium feldspar.
Additionally, metamorphic rocks such as gneiss or augen gneiss showcase banding of light and dark minerals, indicative of their mineral content and the metamorphic processes involved. Knowledge of mineralogy also extends to the creation of pigments from natural rocks with mineral oxides, which produce various colors such as the reddish-brown from iron oxide.