Synthetically grown bismuth crystals. (pic 1, pic 2). Naturally occurring bismuth looks visually uninteresting, but you can grow these colorful, hoppered crystals in the lab. In fact, since bismuth melts at 271 °C, you can make them at home. The colorful exterior is formed when the outside is exposed to air and oxidized; the color varies with the thickness of the oxide layer. The crystal grows faster at the edges than the interior, giving this hollowed-out look.
Bismuth was long thought to be heaviest stable element; as it turns out, Bi is ever so slightly radioactive, but its half-life is a billion times the age of the universe.