If you have a spell with a ‘yes/no’ box, and you choose not to cast that spell, then Harness will still trigger… you keep your original spell full of mana, but you also get a spell back from your graveyard if there is one there, and destroy 3 gems.
I’m not sure this always happens, but it certainly happens regularly.
I just played against it. The opponent can hold a card in hand and it will still trigger the gem destruction.
Similarly, I withheld a spell to prevent a werewolf transformation, and it still transformed. I think the bug is actually spells counting as “cast” and not Harness the Storm.
I’m off to test with more things that count spells.