In The Elder Scrolls V: Skyrim, soul gems are used to trap the souls of creatures and people. These souls can then be used to enchant weapons and armor, or to power certain magical effects. To fill a soul gem, you must first have an empty soul gem and a soul trap spell. Once you have both of these, you can cast the soul trap spell on a creature or person. If the spell is successful, the creature’s or person’s soul will be trapped in the soul gem.
The size of the soul gem determines the strength of the soul that can be trapped in it. Petty soul gems can trap the souls of weak creatures, such as rats or rabbits. Common soul gems can trap the souls of stronger creatures, such as wolves or bandits. Greater soul gems can trap the souls of even stronger creatures, such as trolls or dragons. Grand soul gems can trap the souls of the most powerful creatures in Skyrim, such as mammoths or giants.