It's not a stupid question at all
Do please ask anything you want to now
SPOILER AHEAD
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There are three gems in the game which are worth keeping, you can sell all the others for the cash and buy something nice
The three important gems are:
1. Rogue Stone - you need it to get into a certain building in the bridge district (BE WARNED!!! tough fight!)
2. Diamond - to upgrade (= improve) the silver horn of Valhalla once
3. Beljuril - to upgrade the horn of Valhalla for the second time
You need only carry one of each of the abovementioned gems, so if you have more than one Rogue stone, sell all but one. Hope this helps!
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Melusine, Archbabe of the OHF and the LH
Your voice is ambrosia