If you like collecting interesting things, aerogel should definitely be on your list. Aerogel is one of the most impressive materials humans have created...so far.
This vial contains 25mL of the world's lightest known solid material. Also commonly called "frozen smoke". Safe to touch. Hydrophobic.
Aerogels are a group of materials in which the liquid portion of a gel has been replaced with a gas. It was first created as a result of a bet.
Being the lightest solid material know, aerogels are up to 99.8% air. Its also has the lowest thermal conductivity of any know solid which means it makes a great insulator.
Due to its high porosity, NASA has even used aerogel to collect interstellar dust particles in outer space.