Some weapons can have a mind of their own in New World or so it seems. There are some mechanics that are not apparent at first and here is a look at one of them.
New World weapons have hidden mechanics all over the place but they generally benefit the wielder even if they are not aware of it. One, however, provides a source of frustration until you know its inner workings.
Specifically, this happens when people try to slot a gem that alters a portion of the weapon's damage output into Arcane, Lightning, Frost, Fire, Nature or Void type but then they discover it can't be done.
This happens when a weapon has a different elemental affinity through a perk, such as Chain or Attunement. For example, if it has the perk Chain Fire, you cannot slot a gem on it that would make a portion of the damage switch to Void.
The same goes for weapons that have Arboreal Attunement for example - out of all the elemental gems, you can only slot the ones that alter the damage into Nature because the perk already does it.
Unfortunately, if you bought an Arboreal weapon and you want to slot a gem that would make its damage scale with Intellect, you are out of luck - you will need something with a damage type that corresponds to INT gems, which includes literally any of the other elemental damage types.
It works both ways so if you want your weapon to focus with Focus, you have to get one that has Arboreal Attunement/Chain perk or none of the elemental perks.
On the other hand, if you get a damage increase from non-elemental perks such as Enchanted, Trenchant or Thwarting Strikes, you can slot any elemental gem into the weapon.
Hopefully, this clears up any confusion you might have about weapons and gem compatibility and saves you a few thousand gold in the process.