The Allnatt Diamond is a diamond measuring 101.29 carats (20.258 g) with a cushion cut, rated in color as Fancy Vivid Yellow by the Gemological Institute of America.

This diamond was named after one of its holders, Major Alfred Ernest Allnatt, a soldier, sportsman, art patron and benefactor.

After purchasing the diamond, he commissioned Cartier to make a setting for it.

After being sold to the SIBA Corporation, the diamond was re-cut to its current weight and the intensity was upgraded as a result.

The Allnatt was displayed as part of the Smithsonian's "The Splendor of Diamonds" exhibit, alongside The De Beers Millennium Star and The Heart of Eternity.