I've spent a good few years fantasizing about this ever since I found out about this series and I've come to the conclusion that you'd have to drill a bumpy ribbed hole (that has obviously been smoothed) near their pelvis region and fill it with lube to get a functional gemhole that felt nice and didn't cut your dick.
If this didn't work you could always just drill a onahole slot and stick a pocket pussy in there and fuck the gem like that. Maybe you could have a high tech pocket pussy that sent shocks/vibrations to the gem during sex so that they're also being stimulated as well. Infact you could probably just drill these pussy slots anywhere on their bodies.
Some gems may be too weak to withstand a rough pounding so only a few of them will be able to be fucked like this. So here are some other possible methods of sex with your gem waifu's.
-Hairjobs. Some of the gems have hair that is smooth enough and not sharp enough to be dangerous. So they could probably give hair jobs which don't chafe your dick with the help of lube. then again they may crush your cock
-Handjobs. The fingers on the gems seems pretty delicate and intricate. They may be able to give hand jobs if they wore gloves.
-Powder play. Licking her body and licking her powder off.
-Assjob. Very simple and not dangerous.
-Thighjob. Possible, but may be dangerous because penis may be crushed or cut if thighs are not smooth.
and before you ask, yes I know I am a sad pathetic excuse of a person.