For the correct functionality of the item, I'm afradi you'll have to wait for the next update of tModLoader (sowwry!

).
For now I've made it so that you need to keep the item in your inventory for the additional accessory slots.
Download
P.S.
Sorry it's taking so long to get the build working correctly! ><
I think that you should get +1 accessory slot for each one you have in your inventory. Then it balances the mod by giving you an extra accessory slot, and in exchange taking up one inventory slot. (but of course with a limit as to how many you can have at a time) This also adds a bit more of a challenge to the game, because with less inventory space, you cant hold as many items, which makes farming and mining take longer because you cant hold as much, and you have to go back and forth a lot more... I think the limit for extra accessories should be 4. (because that is 1 line in your inventory not including your hotbar) And also, something people might find handy later on in the game is to have a rare item that extends your inventory by 1 slot. (maybe a 10% chance drop from golem or plantera since they are harder to beat than the mech bosses) And I also think that the names for the items should be:
For accessories: Gemstone Of Configuration
For inventory space: Gemstone Of Expansion
And this also expands all the possibilities for updates in the future if you decide to update this mod, because gemstones are considered precious, and rare, so everyone wants them!
Now I do understand and respect that this mod was supposed to be just a small mod to allow you to equip more accessories, but why stop there? someday this mod might be more popular than the thorium mod if you keep trying to expand its possibilities and make the mod better and better as time passes. If you like my ideas, just ask! If you want to hear some more ideas, I would love to be able to contribute to this project in some way...
-TheDynamicRam
