[10] In the Free Handicap, a ranking of the season's best juveniles Diolite received a weight of 129 pounds making him the best British two-year-old of 1929 ahead of Press Gang (128), Challenger (127) and Blenheim (126).

[12] In the Derby over 1+1⁄2 miles at Epsom Racecourse on 4 June Diolite started 11/4 favourite against sixteen opponents, despite doubts about his ability to stay the distance.

[14] He was among the leaders for most of the way and took the lead early in the straight but was overtaken in the closing stages and finished third behind Blenheim and Illiad, beaten three lengths by the winner.

[15] At Royal Ascot later in June Diolite was dropped back to sprint distances for the five furlong Fern Hill Stakes and finished third behind Stingo.

[16] Diolite returned for the St Leger over 14+1⁄2 furlongs at Doncaster Racecourse but appeared to run short of stamina as he came home seventh behind Singapore.

[18] In November Hirst decline a challenge to send Diolite to Australia for a match race against Phar Lap, suggesting instead that the Australian champion should be sent to England.

[19] Diolite remained in training as a four-year-old but failed to recover his best form and recorded his only victory in a handicap race at Liverpool.