Green’s Dictionary of Slang

of people or objects, first-rate, excellent.

[1834		Ely’s Hawk & Buzzard (NY) Mar. 22 1/3: Tom was considered a diamond of the first water].
1904		Sydney Sportsman (Surry Hills, NSW) 13 Apr. 1/3: Business was booming when ‘long nit’ gave the ‘warehouse,’ and the ‘diamond gee’ repeated it that ‘John Hopper’ was in sight having a screw’.
1926		E. Walrond Tropic Death (1972) 100: The men sung [...] ‘Diamond gal cook fowl botty giv’ de man’.
1936		‘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.
1997		N. Barlay Curvy Lovebox 116: t’xaaaay: diamond cunt.
1999		Guardian G2 26 Oct. 22: A diamond geezer.
2004		N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 478: There goes Razor Smith, he’s a diamond geezer.