This is a reference to Thomas Gray’s ‘Elegy written in a Country Churchyard'. Contemplating the humble rural graves and how their occupants' lives are obscured by the legacy of the wealthy dead, the narrator reflects:
Full many a gem of purest ray serene,
The dark unfathom'd caves of ocean bear:
Full many a flow'r is born to blush unseen,
And waste its sweetness on the desert air.
That Maury applies this meditation on the overshadowed virtues of the poor to the finacial promise of his dubious schemes highlights his gauche character.