It's a fascinating subject, and because the environmental atmosphere in which they are kept can vary so much, it often appears that no two coins are alike. Coins "age" in often very different ways - consider for example "streaky" lustre, even toning or patchy toning. Not to mention coins worn down to VF, but still retaining lustre, or uncirculated coins with lustre so subdued, it's practically undetectable. Also, as Bernie quite rightly says, bronze coins do readily retain their lustre.