Gemstones are one of several currencies that can be found within the Underworld. They can be used to commission cosmetic upgrades to the House of Hades.
||'...Mortals cherish these stones of many brilliant colors, going to lengths to excavate them from the earth; to steal them, to be blunt, from the rightful domain of Master Hades. The Master also values them, for light exists in limited supply down here, and light refracted through a gemstone's facets is most valuable of all. The merest glimpse of such a dazzling display can spur a loathsome shade to action, in the Master's name; and thus has Master Hades found within his realm a naturally-occurring motivator for whatever must be done. And as for mortals who would risk themselves to steal from the earth -- they all tend to forget that, in the span of just a few short years, they shall likely find themselves right here, among the Master's riches that they sought.'
Gemstones can be received as a room clear reward, as a reward for completing Trove Trials, or in trades with the Wretched Broker.