In the Quest with the Demers convection I get the message when I select the warped Gem Stone "You are not able to equip this item'.

I assume you're talking about the Arniel Endeavor quest, so I'll move this to the Skyrim forum.

This is true, you can't equip it. It is just a regular soul gem, which also can't be equipped. What you do is open the convection container and then place the gem into it.

I hope I read your troubles correctly

Welcome to the boards, if not welcomed already