Gems are the main currency in the world of Mareth and are thus required to buy just about everything.
Gems are typically found in small amounts through conventional means. There are only a few ways to get larger amounts in the game.
-Whitney will offer up to 500 (1000 in Easy Mode) gems for cum and milk production every week once a breast milker or cock milker from the Mountain demon factory has been set up (The player can only take one from the factory. Joey at the Bizarre Bazaar will sell a cock milker *only* for 200 gems. A herm player ought to bring the breast milker from the factory for that reason. For this to be viable, an excessively high cum production is required; at least in the thousands of MLs. Cum production can be affected directly as its own stat, and is also increased by ball size. This can be a tedious stat to raise, as there are few relevant items to do so; only fox berries and Succubi's delight directly affect it, easiest found in the desert after the Marcus encounter. (note: for pure characters, an unpurified Succubi's delight will rarely have an effect other than saying "why would you drink that?". Also, if playing on Realism mode, it would be more desirable to reload if the drink increases ball size rather than cum production directly)
-A level 12+, high corruption character [supposedly 70] can use the farm as a cash cow. (link to the vanilla game's wiki)
-Call it an exploit or merchanting, or clever use of game mechanics: Whenever an ascension is performed after slaying the "final" boss, the player will gain access to the tailor shop in Ingnam which sells Classy Suitclothes for 100 each. They can be sold at the pawn shop in the desert city for 200 each. The weapons shop also sells Katanas for a bargain, which may be stored in a weapons rack for additional profit. Profit can be maximised with both "strong back" level-up perks, the large backpack from the Plains' Bizarre Bazaar, the armor rack from the Lake's Town Ruins, and all three chest upgrades (Desert chest -> Swamp exploration unlocks the Bog, Bog chest -> Glacial Rift chest). With all of those storage upgrades, the player could retain a profit of 12,000 gems from the clothes alone in a single trip, enough to buy pretty much everything in the game.
-(Exploit) Benoit (the guy who also conveniently sells the large backpack, granting +5 inventory spaces) at the Bizarre Bazaar will sometimes stock Dragonscales at random, his stock rotating every day. He sells an unlimited amount of them for 12 a piece. Two of these can be crafted, for free and taking no time out of the day, by Rathazul into an undergarment or upper garment which can then be sold for 276 gems profit each. The armor that can be made out of it takes 5 dragonscales, but goes for the same price for some reason. This can be easily exploited for an absurd profit with little effort. Even if not interested in the exploit, dragonscale garments are still decent pieces of gear.
Gems will be stolen upon losing a fight, including some friendly spars such as with the Volcanic Crag's behemoth depending on the level of the opponent. Some opponents take more than others, including said behemoth, who takes several hundred gems at a time. It is uncertain exactly how this works, but it is believed that an estimation of gems lost may be a random number between 1 and 10 plus half their level.