Green’s Dictionary of Slang

1. a diamond, usu. a diamond ring.

1919		T.A. Dorgan in Zwilling TAD Lex. (1993) 73: Its the rhyming slang. Simple simons those are diamonds.
1928		M.C. Sharpe Chicago May: Her Story in Hamilton (1952) 132: Simple Simon – diamond.
1928		R.J. Tasker Grimhaven 180: ‘We took the chump for his Simple Simons,’ a man declares, ‘and made for the shovel and broom.’.
1931		D. Runyon ‘Romance in the Roaring Forties’ in Runyon on Broadway (1954) 34: Well, I do not see any Simple Simon on your lean and linger.
1946		St. Vincent Troubridge ‘Some Notes on Rhyming Argot’ in AS XXI:1 Feb. 47: simple simon. A diamond. (Origin uncertain, probably American.) More probably English. Simple Simon is an old English nursery rhyme.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 194/2: Simple Simon. (Obs.) A diamond tie-pin; any expensive stickpin.

2. (drugs) psilocybin/psilocin.

1970		R. Gilmore Drug Education Hbk.
1971		Illinois Legislative Investigating Committee Drug Crisis in Spears (1986).
2001		ONDCP Street Terms 19: Simple Simon — Psilocybin/psilocin.