Green’s Dictionary of Slang

diamond n.

1. (US short order) a meat pie.

Hawk Eye (Burlington, Iowa) 8 Nov. 3/3: ‘Sleeve buttons,’ [...] is the New York restaurant waiter’s slang for codfish balls. ‘One summer,’ calls for oat meal and milk; ‘two on other,’ is legal tender for two fried eggs; ‘stars and stripes’ means plate of pork and beans; ‘leather and bake’ is liver and bacon and ‘diamonds’ means meat pies.

2. (US gay) in pl., the testicles [play on crown jewels under crown n.1 ].

[UK]Mercurius Democritus 28 May 16: At his first charge with her, quoth he Nan prethee feel my two round Rocks, with which she handled his precious Diamonds [...] and by his direction clapt her hand on his Bum to give satisfaction to the Bumkin, who bid her whip his stinking rotten A—.
[US]Wentworth & Flexner DAS.
[US]Guild Dict. Homosexual Terms 11: diamonds (n.): The testicles; in the sense of the family jewels.
[US]B. Rodgers Queens’ Vernacular.

3. a first-rate person or object [note earlier SE form].

[[UK]C.M. Westmacott Eng. Spy II 47: Her next paramour was a diamond of the first water, but no star, a certain dashing jeweller, Mr. C---].
[UK]G.F. Newman A Prisoner’s Tale 106: He decided the screw was a diamond after that.
[UK]J. Sullivan ‘Homesick’ Only Fools and Horses [TV script] He’s a diamond, he really is.
[UK]Indep. on Sun. Rev. 20 June 57: It was a diamond. [...] It had so much energy and was so strong, funny, clever, new and original.

SE in slang uses

In compounds

diamond-cracking (n.)

breaking rocks as part of one’s prison sentence; thus diamond-cracker, one who is working off such a sentence.

[UK]Bradford Obs. 6 Dec. 6/6: ‘If you’d been here on a week-day you’d hev hed to crack diamonds to the tune of two bushels.’ [...] ‘Cracking diamonds’ is the slang term for breaking stones.
[UK]Dundee Courier (Scot.) 21 June 7/5: A sailor, whose heap of unbroken stones plainly indicated that he was novice at ‘diamond-cracking’.
[UK]Aus. Printer’s Keepsake n.p.: He caught a month, and had to white it out at diamond-cracking in Castieu’s Hotel [Melbourne Gaol] [F&H].
diamond-cutter (n.)

the erect penis.

[US]J. Wambaugh Choirboys (1976) 204: ‘Did you have a blue veiner?’ ‘A diamond cutter as a matter of fact.’.
[UK]Roger’s Profanisaurus 3 in Viz 98 Oct. 10: diamond cutter n. The hardest erection known to man.
[UK]P. Meditzy ‘A Day In The Life Of...’ 29 Apr. [Internet] At the same time she put her hand in my pants and started to ‘choke the chicken’ and then on to ‘smoking the Whitehouse cigar’, taking it deep into her throat until my cock was like a ‘diamond cutter’.

In phrases