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Luster is how the light reflects off of a mineral. If the mineral has a metallic tint and is completely opaque (even around the edges), then it has a metallic luster. Examples include:
CopperPyriteGalenaChalcopyriteSpecular Hematite
copper pyrite galena chalcopyrite specular hematite
If a mineral does not look like a metal and/or lets even a little light through (even just around the edges), then it is said to have a nonmetallic luster. Specimens can be further classified into different types of nonmetallic luster, but this can get somewhat subjective:

Example Luster
Hematite Earthy (dull, gritty or soil-like)
Quartz Vitreous (Glassy)
Opal Pearly
Sphalerite Resinous
(Some resinous minerals might seem metallic at first, but look closely: their surfaces seem a little more like overly-tinted glass than like metals.)
Talc Greasy
(These reflect light the way waxed paper does. Some minerals with a greasy luster, like talc, actually feel greasy, too.)
Ulexite Silky
Topaz Adamantine (diamond-like)

Text Source: Rocks and Minerals, by Charles A. Sorrell (C)1973 Western Publishing Company Inc.
Illustrations: Personal Collection of the Author

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