Gemstones are one of several Artifact currencies that can be found within the Underworld. They can be used to commission upgrades at the House Contractor. It's currently the bottom tier artifact in the "trade up" tree.
'...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.'
Chamber Rewards : Blue Laurels chambers can give Gemstones as Bounty, with three possible amounts depending on the type of room. Normal Rooms give 5 Gemstones, Hard Rooms give 10 Gemstones. Blue Laurels chambers will be more common when Bosses drop their bounty instead of Darkness.
Infernal Troves : Once purchased from the House Contractor, some rooms will contain an Infernal Trove which sometime contains Gemstones. Infernal Troves are challenges where the player must slay foes quickly to claim a bigger part of the reward. The troves can be upgraded to contain more Gemstones, from 25 to 50 and finally 75 , for a total investment of 660 .
Fishing : Selling fishes caught in Tartarus and Styx to the Head Chef rewards with gemstones based on the rarity of the fish. The common fish are of little value compared to the rare ones, making the easy mastery of the fishing minigame a reliable entry of Gemstones. The chance of spawning fishing spots can be increased with Poseidon's boon Huge Catch.
The fish caught in Tartarus are worth 5 / 20 / 30 .
The fish caught in Styx are worth 20 / 40 / 150 .
Vanquisher's Keep : Getting the Vanquisher's Keep order from the House Contractor for 4 makes Bosses also drop Gemstones as a reward. Each drop increases in value with the difficulty. Furies 10 ; Hydra 20 ; Heroes 30 ; Hades 50
Ocean's Bounty : This Poseidon boon drops a pile of , , and in varying proportions.