"If magical girls are said to be born from positive desires like wishes, then witches are born from negative desires like curses."
"Magical girls spread hope, where witches spread despair."
"And to make matters worse, they can't be seen by normal humans."
"Doubt and suspicion, extreme anger and hate. They bring seeds of catastrophe to this world."
— Kyubey, episode 2.
The soul gem is the source of a magical girl's magical powers; but, every time the magical girl uses her magical powers, the soul gem becomes "tainted". When "tainted", the soul gem loses its lustre and becomes more and more dark. The color of the soul gem darkens also. The only way to rid the soul gem of its "taint" is to use a grief seed. The grief seed will absorb the "taint" out of it.
When a magical girl transforms, the soul gem transforms together with her and becomes a part of her costume. The magical girl can also transform the soul gem into a ring, which makes it easier for the magical girl to take it with her and not lose it.
Episode 6 reveals that soul gems contain the magical girl's soul, while the body is merely a husk that is controlled by the gem.
Episode 8 reveals if the soul gem is not cleansed in time and reaches a certain point, it will become a grief seed and birth a new witch. Which, in other words, means that the more "tainted" the soul gem gets, the bigger the risk of becoming a witch.
See this page for list of soul gems in the franchise.
All grief seeds drain grief when used on a dim soul gem, cleaning it. In episode 6, Kyubey warns that absorbing too much grief can revive the witch.
Sometimes magical girls also use grief seeds to test if she can cooperate with another magical girl. Rejecting a grief seed from another magical girl means refusing to work with her and hostility. Both Mami and Homura did such kind of tests.
For the fake grief seeds known as "evil nuts" in the Kazumi spin-off manga, please see here.
See this page for list of grief seeds in the franchise.
Episode 8 reveals that witches are in fact the "grown" forms of magical girls. See also:
As revealed in Magia Record, witches are capable of, and can be made stronger by, consuming Grief Seeds and other witches.
In the anime, these locations are designed and animated by Gekidan Inu Curry in a style vastly different from that of the rest of the series in order to reinforce the otherworldly aspect. Inside and outside of the anime they are sometimes referred to as "mazes" (迷路 meiro) or "wards".
According to the January 2012 Dengeki PlayStation, the barrier is "a space resembling the magical girl's mental landscape before she became a witch".
The barrier can be based off a lot of things, such as where the girl became a witch, what their soul gem or grief seed motifs are or were, what they liked, and/or what their wish was.
This can be proved by looking at Oktavia von Seckendorff's barrier. It has a theme of longing for Kyosuke, since Sayaka's wish was to heal his hand. The the ulterior motive behind the wish was for Kyosuke to love her, but this didn't happen. There is a Kyosuke-like figure shown in her barrier. Additionally, she became a witch at a train station, and at the beginning of episode 8 we see trains in Oktavia's barrier.
They do not drop any grief seeds when killed unless they have grown into a full witch.
According to the January 2012 Dengeki PlayStation, familiars are "projections of the magical girl's heart before she became a witch".
From left to right:
- The cards for Kriemhild Gretchen and Walpurgisnacht do not actually depict their kisses, but the order presented throughout the top two rows matches that of appearance in the anime. The imagery presented also correspond to their themes.
- While two different kisses appear on Homulilly's Rebellion-based cards, her card based on the Portable game does not depict any kisses to speak of, but the form shown of an hourglass corresponds with her theme of time.