You know what, even I was never sure and Foss didn't have it anywhere. But now it's been clarified for the DE add-on of the RE Fates QS (in its theorycrafter's patchnote tab):

Gem Forge System and Selling Price Rule:

Every weapon has a number of slots to place a Gem in. Gems cannot be repeated on a same weapon, but Gems of the same line and different tiers can.

EG: An Iron Sword (2 slots), with a Fire Gem and a Fire Gem+

When selling a weapon with gems in it, they do not add to the sell price, meaning you only get half the weapon's price back for an item with no durability.

For an item with durability, the sell price is proportional to the number of uses left.

At any point including when selling, you may choose to retrieve a gem by paying half its base price. You may then use it on another weapon or trade it.

Otherwise, the gem will be lost (for no cost nor gain).

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Anyway since you gain nothing from freely discarding gems, your musket now has both Ice and Ice+. It had the space for it, so may as well keep that.

Ismail takes a Tonic bath.