Faire (or, payer ) rubis sur l'ongle = To pay to the last farthing.

[This expression means literally to drain a tumbler so completely that there just remains in it one drop of wine, which being put on the nail looks like a ruby.

“Je sirote mon vin, quel qu'il soit, vieux, nouveau; Je fais rubis sur l'ongle, et n'y mets jamais d'eau.” REGNARD, Folies Amoureuses, iii. 4.]