How about a postage stamp? In terms of size and weight, no other natural or man-made object is more valuable than the rarest of the rare stamps.
The world’s most expansive stamp was printed in Sweden in 1855 and was the result of a painting error. Instead of printing the Treskilling stamp on green stock, it was printed on yellow / orange stock paper. Only one copy of Treskilling Yellow postage stamp is known to exist. It first achieved a million dollar price tag when it was sold in 1990. Six years later, it was sold for 2.5 million Swiss Francs, around 2.3 million US dollars. In 2010, the stamp made headlines again with a record-breaking sale. While the exact figure is unknown, the auctioneer revealed that it at least maintained the $2.3 million price achieved in 1996.
Treskilling Yellow (Wikipedia)