Give a mineral specimen to a nongeologist and ask them to describe it. Generally, they mention the appearance, especially color, first. With a little prodding, they may go on to describe the shape and nature of visible crystals. For example, they might describe the pyrite crystals in Figure 3.3 as metallic, having a gold, and forming cubic crystals. They might describe the rose quartz in Figure 3.4 as hard, pinkish, glassy, and partially transparent.
Metallic and glassy are terms describing luster. Gold, clear, and pinkish describe color. Transparent describes diaphaneity. Cubic describes crystal shape, a property related to symmetry. These four properties (luster, color, diaphaneity, and shape) are basic for mineral identification. Other properties including streak (the color of a mineral when powdered), the way a mineral breaks (cleavage, parting, fracture), and hardness are also common keys to identification. Still, other properties can be important for specific minerals.
Given a single property, for example luster, we can sort minerals into groups. In the case of luster, we usually start by dividing minerals into those that are metallic and those that are nonmetallic. The pyrite seen above is metallic. The quartz is nonmetallic. There are, however, many metallic and many nonmetallic minerals; other properties must be considered if minerals are to be identified correctly. Nonmetallic minerals can, for instance, be divided further based on more subtle luster differences.
Ultimately, we can identify minerals by name or at least place them into small groups based on their properties. It is tempting, then, to come up with a standard list of properties that we should evaluate when identifying minerals. However, most mineralogists know that, depending on the sample and circumstances, some properties are more important than others. Rather than going through a long list or filling out a standard table, experienced mineralogists focus on the properties that are most exceptional or unique. Sometimes, a single property, such as strong effervescence by hydrochloric acid (diagnostic of calcite), may serve for mineral identification. And, being magnetic usually identifies magnetite. (Metallic iron in meteorites is also magnetic.) At first, mineral identification may seem tedious, but with a little experience, it is possible to find shortcuts to make the process more efficient.