United Nuclear, sellers of dangerously strong neodymium magnets, also 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.
Link (Thanks, Mike!)
Reader comment: Eric says:
United Nuclear is a really cool site. I recently bought some powerful neodymium magnets, and they were a lot of fun. But not from United Nuclear. What they sold for $75, someone else was selling for under $17, and most of their other prices, at least on magnets, are similarly inflated. Nobody else lists the larger super magnets, but the other sites I saw have a custom order sheet.
After a little comparison shopping, I went to magnet4less.com to get a 1" thick by 2" diameter N40 magnet, among other things. I do not have a united nuclear magnet to compare, but I was happy with what I got at a fifth the price. I have never purchased anything from united nuclear, so other than their prices and blinking web pages, I have no complaints.
United nuclear is a great clearing house of fun (dangerous) toys, but the price is not right. Shop around first. A quick search did not, in fact, reveal any other vendors of aerogel. And in the nasa articles it sounded somewhat expensive, like $1000 per liter. I expect someone else will have it for sale at less than half the price soon though.
Just a warning note, if you want to make a bracelet, get the magnets that are NOT magnetized through the thickness. I made that mistake, and while still fun, the little magnets I got did not like the bracelet shape. At magnet4less you have to custom order cylinders magnetized through the diameter, while other sites have caught on to the bracelet fad.
United nuclear also list grade N45 as the strongest grade, while other sites have N50 grade magnets. And the other sites do not require the signed disclaimer, which sounds like marketing hype. Even my smaller magnet could hurt someone if not respected, but a signed disclaimer? Anyway, enough debunking.