So the crystalarium is pretty neat once you get the hang of it - you can craft one when you hit mining level 9 using some stone, gold bars, iridium bars and a battery pack. If you're not quite there yet mining-wise, you can also snag one by giving the museum 50 different minerals or finishing up that vault bundle at the community center.

Once you've got it running, it's basically like having your own personal gem machine. I usually keep a mix going because different gems are useful for different things. Like diamonds are great if you want pure profit since they sell for 750g each, but i've found jade to be super helpful when you're planning some skull cavern runs - you can trade them for staircases at the desert trader on sundays. Ruby's another good one, especially if you want some spicy eel buffs for mining trips.

Something interesting i learned the hard way - you can't duplicate prismatic shards in these things, but pretty much any other gem works fine. I actually keep a few crystalariums running with quartz just to have a steady supply of refined quartz for crafting, and it works out pretty well. The nice thing is you can always pop out whatever gem you're duplicating and switch to something else if you need to, which makes them really flexible. Some people make huge setups with these things but even just having a few around is super helpful for keeping your gem supply topped up.