The rarest and most expensive of all gemstones is red diamond. In fact there are only perhaps 20 to 30 red diamonds known to exist, and most are less than half a carat. When offered at auction, they typically sell for more than $1 million a carat. The largest red diamond known is the 5.11 carat Moussaieff Red Diamond, a trillion cut from Brazil.
This diamond was reportedly found by a Brazilian farmer in the mid-1990's as a rough stone of about 11 carats. The diamond was purchased and cut by the William Goldberg Diamond Corp., where it went by its original name, the Red Shield.
(Information shared from GemSelect.com)