This is a pure bismuth crystal. The heaviest element that is not radioactive (ok technically it is but it’s half life is like 9 orders of magnitude older than the universe so it really doesn’t count.) Probably my favourite crystal structure, even if you forget the colour. Surprisingly, bismuth is also super-not-toxic. You can actually eat the stuff and it’s often in indigestion remedies. Fascinating element, all round.
Osmium is both the densest and the rarest element that you can dig out of the Earth. It's so hard that it's used (as an alloy) in places with constant pressure and wear, like the tips of fountain pens. One single cubic inch of this stuff weighs nearly a pound, and were you to somehow get a cubic foot of it, it would weigh over 1,400 pounds. Good luck with that, though: last year, the entire U.S. osmium production was only 165 pounds.