Green’s Dictionary of Slang

diamond adj.

[the value of the precious stone]

of people or objects, first-rate, excellent.

[US]E. Walrond Tropic Death (1972) 100: The men sung [...] ‘Diamond gal cook fowl botty giv’ de man’.
[Aus]‘Banjo’ Paterson Shearer’s Colt 201: We’ll take care of him, and, oh boy, what a diamond price we’ll get about the Frenchman next Saturday.
[UK]N. Barlay Curvy Lovebox 116: t’xaaaay: diamond cunt.
[UK]Guardian G2 26 Oct. 22: A diamond geezer.
[UK]N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 478: There goes Razor Smith, he’s a diamond geezer.