Luster describes how a mineral reflects light from its surface, such as metallic, vitreous (glassy), pearly, silky, resinous, or dull. It is easiest to judge on a clean, unweathered face. A dusty coating or oxidation film can mute luster, so wipe the specimen or look at a fresh chip. Collectors use luster to separate look-alikes, like metallic sulfides versus dark oxides or glassy quartz versus dull chert.
Judge Luster under steady light (daylight if possible) and compare multiple faces because some minerals show different luster on cleavage faces versus broken surfaces. If the specimen is wet, dry it first—water can make dull material look artificially glassy. Note mixed luster honestly (for example, “vitreous with dull coated patches”) because that can indicate alteration or weathering. When photographing for records, include one close photo that shows how the surface reflects light.
Luster vs. wet shine Water can make dull minerals look glassy. Dry the specimen before deciding on luster.
Luster vs. surface coating Iron oxide films and dirt can hide true luster. Check a fresh chip or a cleaned face.
Luster vs. transparency A mineral can be glassy and still opaque, or dull and translucent. Record luster and transparency separately.
Luster vs. “sparkle” from tiny grains Glitter from grainy texture is not the same as a glassy surface. Look at a single flat face when possible.