Soul Gem

Soul Gems are a holding place for a magical girl's soul and are the source of both their powers and battle gear.

An incubator explains that the body is just a container possessed by a soul. They create a magical girl by removing this soul and placing it into an external container called the Soul Gem. This revelation horrifies the magical girls, who did not understand the repercussions of their contract and considered their new state to be zombie-like. As their souls are now within these gems, their body will become inert if the Soul Gem is outside the range of 100 m. Only returning to the gem to within this range will resuscitate the magical girl's body.

Darkness spreads into a Soul gem when a Magical Girl uses their powers, or experiences despair and similar negative emotions. Although using a witch's Grief Seed can reduce this corruption, it is inevitable that a Soul Gem will be fully corrupted and transition into another Grief Seed. The affected magical girl will then transform permanently into a witch.

After the establishment of the Law of Cycles, the witch that forms from the Magical girl is prevented from forming in the first place, which causes Magical Girls to instead simply fade away.