Rainbow Bismuth

Bismuth is one of the pure chemical elements (Bi, 83) and a metal with many unique properties. It is almost as dense as lead, but without the toxicity. It is also one of the least thermally conductive metals, but has a relatively low melting point (271°C, 520°F). In fact, when melted, bismuth can crystallize upon cooling into the stair-stepped pattern. The iridescent colors are caused by the oxidation of the outer layer as it cools.

Bismuth was recently discovered to be very weakly (harmlessly) radioactive. But since its half-life is longer than the age of the universe, it can be considered stable. Bismuth is a key ingredient of Pepto-Bismol. You can even extract it from the pink medicine with a little chemistry.