The World of Rare Stones: Painite is the World's Rarest Gem
A more widely recognized (but still very rare) gem is the red diamond. There are many more red diamonds available than some other rare gems, but very large red diamonds are extremely rare. The largest cut natural red diamond may be the one owned by Moussaieff Jewellers Ltd, a 5.11 carat trillion cut.
The rare blue diamond, made famous by the Hope Diamond, gets its deep blue color from traces of boron in the stone. It is a natural blue diamond, in contrast with most of today's blue diamonds, which are turned from clear to blue with a process involving irradiation and heat.
The Hope Diamond was believed to be originally over 112 carats, but was poorly cut. It was recut in the 1600's, resulting in a stone just over 67 carats. It changed hands many times, and got its name from Henry Philip Hope, who acquired the diamond in 1839. After it changed hands a few more times, the Hope Diamond came to rest in the care of the Smithsonian, and now weighs over 45 carats.