Green’s Dictionary of Slang

(UK Und.) of jewels, of the highest quality/value.

1809		G. Andrewes Dict. Sl. and Cant n.p.: rum or monogen good, the most valuable of any thing; jewels, diamonds.
1821		Flash Dict.
1835		G. Kent Modern Flash Dict. 29: Rum, or monogin – good, the most valuable of any thing jewels, diamonds.
1848		Flash Dict. in Sinks of London Laid Open [as cit. 1835].