Item from madoka-magica by
A magical girl's crystallized soul, which powers her magic and transforms into her gem form.
The Soul Gem is a small, ornate jewel that glows with the color of the magical girl's magic. It is worn as an accessory and darkens as magic is used, accumulating impurities that must be purified with a Grief Seed. If it becomes completely dark, the gem cracks and the girl dies.