We had a discussion a while back on AWAD about the density of minerals. Iridium, according to Stales, I'm pretty sure, was the most dense. I can't recall which was the least dense, although ice was not very dense--there was something less dense than ice, however...

I guess you wouldn't want a pen with a tip made of ice!

I suppose I should do a search here on "iridium" to get my facts straight...