Final answer:
Color is not the best method to identify minerals due to variations caused by impurities. The streak, or powdered color, of a mineral is more reliable for identification. In fine-grained rocks, the overall rock color, rather than individual mineral color, can indicate the rock type.
Step-by-step explanation:
Color might not be the best method to identify minerals because the color of minerals can vary widely due to small impurities in their chemical composition.
For example, quartz can appear in multiple colors due to trace element substitutions in its atomic structure, thus not serving as a reliable indicator. Furthermore, mineral composition, which is a more accurate way to identify minerals, oftentimes requires microscopic observation unless the rock features large crystals, known as phenocrysts.
Moreover, the streak of a mineral, which is its color in powdered form, is often more consistent and can be a better identification tool, especially for metallic minerals.
Streak is observed by rubbing a mineral across a ceramic streak plate and can reveal a color indicative of the mineral's true identity, unaffected by superficial impurities.
To identify minerals in fine-grained extrusive igneous rocks where grains are too small to see, geologists rely on the overall color of the rock itself. For instance, mafic and ultramafic igneous rocks, rich in iron and magnesium, appear darker, while felsic igneous rocks, rich in potassium, aluminum, and silica, are lighter in color.
These color differences in the rocks, rather than individual minerals, can help geologists determine the rock type.