While looking at Soul Gems in the CS, pondering the possibilities, I noticed that there isn't anything different about them, at least not as far as categorization/labeling is concerned. They're Miscellaneous Items, but then again so are a lot of other things, which of course do not enable one to do any soul harvesting. So what exactly is it that the game checks for in the player's inventory that lets it know that they have an adequate Gem for the soul they just successfully trapped?