Diamond was bred by Francis Dawson from a Matchem mare he had acquired from her breeder, Richard, 1st Earl Grosvenor. He was sold to Joseph Cookson at the end of his four year old campaign in 1796. Joseph Cookson also bred the Derby winner Sir Harry (br c 1795 Sir Peter Teazle) who was exported to America in 1804. After Mr Cookson's death in 1799, his executors ran Diamond in his engagements for 1800.

He was described as a compact, strong horse of large bone and thought to be the stoutest horse of his day in the kingdom. A durable runner, he raced for six years beating some noteworthy rivals, including the Oaks winner Hermione (b f 1791 Sir Peter Teazle) and Stamford (br c 1794 Sir Peter Teazle), who twice won the Doncaster Cup.

However, Diamond's most famous race was a match with Hambletonian (b c 1792 King Fergus) which took place on Monday, March 25, 1799, at the Newmarket Craven Meeting. Diamond represented the south, or Newmarket,  with the Yorkshire horse Hambletonian representing the north. The match may have reflected the rivalry between the two top sires, Highflyer and King Fergus, as well. Diamond, ridden by Dennis Fitzpatrick carried 8 stone, and Hambletonian, ridden by Frank Buckle, carried 8 stone 3 pounds. The match was for 3000 guineas a side, half-forfeit, with large amounts said to have been wagered on the outcome. Run over the Beacon Course of 4 miles, 1 furlong and 138 yards, the time was given as 8 minutes and a half. Although the game little Diamond lost by a neck he was still thought the better horse overall as he ran in more races and beat a higher quality of opponent than did Hambletonian.

While Diamond made no great impression in the stud he did leave two daughters who bred on. Queen of Diamonds (b f 1809) was the 2nd dam of the Oaks and St Leger winner Queen of Trumps (br f 1832 Velocipede) and ancestress of such horses as the Chester Cup winners Organist (b c 1871 Cathedral) and Hidennis (b c 1921 Tredennis) and the good stallion Chop Chop (b c 1940 Flares) who was such a success in Canada. Another daughter, Miss Forester [ex-Miss Holland] (b f 1808), bred by Sir Stephen Glynne, was the dam of Mr Houldsworth's Valentine* [ex-Tommy Longlegs] (b c 1823 Magistrate) who was sent to New York in 1826.

Several old sources say Diamond was sent to America and "created quite a sensation in Dublin when he passed through that city on his way to his new home," however, there is no record of Diamond being in America, and he is also said to have been sent to France in the spring of 1818 [GSB 2/2:312]. He is listed as a sire of winners In Ireland in 1810 [Pick:1810]. Diamond is said to have died in France in 1819.