I need help about spirit gems

Hello! I've been playing for few hours and I'm at level 27 in act 2 playing a witch. At the beginning I was playing the game without a guide and a Friend told me I should follow one to help progress and not being too frustrated by the game difficulty.

So I have it a try and started follow this guide to realign my witch : https://maxroll.gg/poe2/build-guides/minion-army-infernalist-leveling-guide

Maybe it's a good one or a bad one but it's not my question. Apparently in act 1 there is a boss called The king in the mists, that's supposed to drop an uncut spirit gem.

I beat that boss before going to act 2 but I can't find that gem anywhere. It's not in my inventory. Not in my stash. Nowhere. When I'm looking for info about this boss, such as in the following IGN page https://www.ign.com/wikis/path-of-exile-2/The_King_in_the_Mists There is no mention of this spirit gem.

Some people are talking about it when I'm looking at search engine results but the issue remains the same for me. I don't have it in my inventories.

Could I have used it without knowing ? How to check that ? I don't see anything particular in my passives skills tree page. I don't see anything particular in the active skills page either.

Would it be possible that I forgot to loot it ? Or sold it by mistake ?

How can I solve this situation ?

Thanks in advance for your help.