Green’s Dictionary of Slang

diamond n.

1. [late 19C] (US short order) a meat pie.

2. [mid-17C; 1960s] (US gay) in pl., the testicles [play on crown jewels under crown n.1 ].

3. [1970s+] a first-rate person or object [note earlier SE form].

SE in slang uses

In compounds

diamond-cracking (n.)

[mid-19C–1910s] breaking rocks as part of one’s prison sentence; thus diamond-cracker, one who is working off such a sentence.

diamond-cutter (n.)

[1970s+] the erect penis.

diamond-squad (n.) [their jewellery]

[mid-19C] (UK Und.) rich, powerful individuals.

In phrases

diamond a horn (v.) [fig. use of a SE diamond for a stone + ? horn as the shape of one’s lame leg]

[mid-19C] (UK Und.) to place a stone in one’s shoe to counterfeit lameness.