Minerals with dull luster reflect very little light. Greasy luster looks similar to the luster of solidified bacon grease. Silky means relecting light with a silk-like sheen. Pearly luster refers to a subtle irridescence or color play in the reflected light, same way pearls reflect light. Pearly luster is important in identifying the feldspars, which are the most common type of mineral. Of the nonmetallic lusters, glassy is the most common and means the surface of the mineral reflects light like glass. For example cleaned polished pieces of chrome, steel, titanium, copper, and brass all exhibit metallic luster as do many other minerals. A metallic luster means shiny like polished metal. It is often useful to first determine if a mineral has a metallic luster. Standard names for luster include metallic, glassy, pearly, silky, greasy, and dull. For example, a mineral described as “shiny yellow” is being described in terms of luster (“shiny”) and color (“yellow”), which are two different physical properties. It is not the same thing as color, so it crucial to distinguish luster from color. Luster is how the surface of a mineral reflects light. Minerals such as quartz have a non-metallic luster. Minerals that are opaque and shiny, such as pyrite, have a metallic luster. One simple way to classify luster is based on whether the mineral is metallic or non-metallic. Mineralogists have special terms to describe luster. Luster describes the reflection of light off a mineral’s surface. Purple quartz, known as amethyst, and clear quartz are the same mineral despite the different colors. Color is readily observable and certainly obvious, but it is usually less reliable than other physical properties. Some characteristics, such as a mineral’s hardness, are more useful for mineral identification. The physical properties of minerals are related to their chemical composition and bonding. Minerals can be identified by their physical characteristics.
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First, you need to become familiar with the physical properties of minerals and how to recognize them. The chemical formula and crystal lattice of a mineral can only be determined in a laboratory, but by examining a mineral and determining several of its physical properties, you can identify the mineral. Together, the chemical formula (the types and proportions of the chemical elements) and the crystal lattice (the geometry of how the atoms are arranged and bonded together) determine the physical properties of minerals. Although thousands of minerals in the earth have been identified, just ten minerals make up most of the volume of the earth’s crust-plagioclase, quartz, orthoclase, amphibole, pyroxene, olivine, calcite, biotite, garnet, and clay.
Minerals are naturally occurring, inorganic solids with a definite chemical composition and a crystal lattice structure. When identifying a rock you must first identify the individual minerals that make up that rock. (Obsidian is a volcanic rock made of glass and coal is made of organic carbon.) Most rocks contain several minerals in a mixture characteristic of the particular rock type. All rocks except obsidian and coal are made of minerals.
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