Bismuth

Bismuth is a heavy, silvery-white metal with one more proton in each nucleus than lead. Despite its close physical similarity to lead and chemical similarity to arsenic, it's virtually nontoxic, and appears in a variety of cosmetics, pigments, and pharmaceuticals. You can read more about it here:
Bismuth

For the better part of a century bismuth was thought to be the heaviest stable element, but its only naturally-occurring isotope is now known to be radioactive. Granted, the half-life is about 12 quintillion years, but radioactivity is radioactivity, and eventually every atom of bismuth will decay through beta emission into lead, which is now known to be the heaviest stable element. There are more facts about bismuth, but the real reason I'm writing about this is to justify the cover photo. As it turns out, bismuth surfaces easily oxidize into iridescent color patterns, much like oil slicks. The physics of this is interesting, and the imagery is simply, wonderfully, beautiful.