Green’s Dictionary of Slang

(UK Und.) a jewel, a gold ring; thus rum-jem, a diamond ring.

1725		New Canting Dict. n.p.: jem a Gold Ring; Rum-Jem, a Diamond one.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
a.1790		H.T. Potter New Dict. Cant (1795) n.p.: jem a gold ring.
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1833		‘The Slap-Up Cracksman’ in Swell!!! or, Slap-Up Chaunter 43: So flick the suck — or draw the clicks, / The lil, the jam, or bung from kicks.
1848		Flash Dict. in Sinks of London Laid Open 112: Jam, gold ring.
1859		Matsell Vocabulum.
1881		Trumble Sl. Dict. (1890).
1895		C. Crowe Aus. Sl. Dict. 349: Jem,, a gold ring.
1903		Bulletin (Sydney) 2 July 36/1: ‘You hold it and I want it. I’ll sling you ten jems to-morrow, and if you don’t part now I’ll smash you.’ / ‘If you smashed me to dust and riddled what was left you wouldn’t get a sprat.’.