Luster is a term used to describe the way light interacts with the surface of a crystal, rock, or mineral). It is a physical property used by mineralogists to help identify minerals. The word "luster" traces its origins back to the Latin word "lux," meaning "light," and generally implies radiance, gloss, or brilliance). There are different types of luster, and the terms used to describe them include earthy, metallic, greasy, and silky, among others). The following are some of the types of luster:
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Metallic: This is the lustre of metals, such as gold, tin, and copper. Minerals with a metallic lustre are usually opaque and have refractive indices near 2.5.
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Vitreous: This is the lustre of broken glass and is the most common lustre in the mineral kingdom. It occurs in translucent and transparent minerals with refractive indices between 1.3 and 1.8, such as quartz.
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Adamantine: This is the nearly metallic lustre of diamond and other transparent or translucent minerals with high refractive indices (between 1.9 and 2.5) and relatively great density, such as cerussite and other compounds of lead.
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Resinous: This is the lustre of yellow resins, such as sphalerite.
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Greasy: This is the lustre of oiled surfaces, such as nepheline and cerargyrite.
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Pearly: This is the lustre that looks like pearl or mother-of-pearl, such as muscovite.
Lustre varies over a wide continuum, and so there are no rigid boundaries between the different types of lustre. The terms are frequently combined to describe intermediate types of lustre (for example, a "vitreous greasy" lustre)). It is important to note that luster is not a diagnostic property, and it can vary from one specimen to another. Therefore, it cannot be heavily relied upon in mineral identification, but it can be considered a "hint" that can set a person on the proper route.
