Aerogel: lightest solid known to exist
United Nuclear sells chunks of aerogel, a form of silicon dioxide with 1/1000th the density of sand (which is also made of silicon dioxide).
Aerogel (also called ‘frozen smoke’ because of its hazy blue appearance), is a truly remarkable material. It is the lightest and lowest-density solid known to exist, and holds an unbelievable 15 entries in the Guinness Book of World Records, including best insulator and lowest density solid.
Aerogel is composed of 99.8% air and is chemically similar to ordinary glass. Being the world’s lightest known solid, it weighs only three times that of air.
Only $125 (R750+) for a funny-shaped piece, what a bargain. Beats me what you’d use it for, since it won’t make much of a paper weight, but something that cool doesn’t need a purpose.
Update: Looks like you can’t buy it anymore, but here’s an article on maltaStar that details Aerogel’s development and new uses that are being found.
Update 2: Some photos at the NASA website show aerogel’s abilities as an insulator, by blasting crayons on top of a piece of aerogel with a blowtorch.