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Message:: [QUOTE="-jeffB, post: 1867856, member: 27832"]It seems like you don't understand the definition of "luster". It doesn't mean "metallic shininess". It means "reflectivity that depends on the direction of the light source". That anisotropic (different in different directions) reflectivity is what causes the "cartwheeling" effect characteristic of luster. Brown coins can most definitely exhibit it, even if the reflections aren't as bright as those from a red coin. It comes from flow lines on the surface, which are so large that they can be seen under an ordinary optical microscope; the surface layer that causes red copper to turn brown is much, much thinner. You also seem to be under the impression that slabs are air-tight. They aren't. Another member here has told of seeing a coin doctor tone a coin very quickly without removing it from the slab, presumably by sucking the air out of the slab and replacing it with a reactive gas.[/QUOTE]