Soul Gems

Soul Gems are created when a girl makes a contract with Kyuubey and are proof that someone is a Puella Magi and also allows the transformation into Puella Magi form to take place. It is eventually revealed that they are exactly what their name suggests: gems that contain the souls of Puella Magi so that their weak human body may be more protected from the full effects of fighting Witches. When a Soul Gem becomes corrupted by despair and the overuse of magic, it becomes a Grief Seed and the Puella Magi the Soul Gem belongs to becomes a Witch herself.

Appearance
When in human form, Soul Gems can be held, but they can also take the form of rings with the series' trademark runes on it. One of the girl's fingernails also gets a simple pattern put on it. Each Soul Gem has a unique appearance depending on the Puella Magi it belongs to. The basic shape is the same, but the color of the gem depends on the main color of the Puella Magi's transformed outfit and on the top of the Soul Gem is a symbol or shape different for each girl. In Puella Magi form, the Soul Gem manifests itself as a piece of jewelry (like a hair clip or broach) on the girl's costume, in a form similar to the shape on the top of the Soul Gem.