Red-letter day as gemologists discover why crimson diamonds are so rare

The Winston Red, one of only 24 red diamonds of more than one carat publicly recorded, is on display in Washington DC

Red diamonds are some of the rarest gems on the planet: only 24 stones of more than one carat (200 milligrams) have been publicly recorded.

Now, one of the finest red diamonds - the Winston Red - has gone on public display at the Smithsonian National Museum of Natural History in Washington DC.