Because the potion in my game used to look more magical than the stone I use to store souls. 

The empty soulgem have daedric runes glowing and moving inside the rings
The full soulgem have faster rune that glow more brightly and the gem have a cloudy effect

The gems have been renamed for better sorting:
Soul Gem - Azura's Star
Soul Gem - Black
Soul Gem - Black Star
Soul Gem I - Petty
Soul Gem II - Lesser
Soul Gem III - Common
Soul Gem IV - Greater
Soul Gem V - Grand

Install with your favorite mod manager