YinYang1st |
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Spell Gems specify that you need a "high enough key ability score to cast it." Given that they don't specify what score to use, I would assume it's the ability score associated with whichever class's spell list the spell belongs to. So, if it's a Mystic-only spell, I would think you can't use INT and must use WIS like normal.
An exception to this exists in the Envoy Improvisation: Spell Gem Understanding. It specifies that you "treat all spells on the mystic, technomancer, and witchwarper spell lists as your class's spell list, and you use Charisma as your key ability score for spellcasting."