Grief Seeds are black egg-shaped thorns created at the birth of a witch. Once planted, they form barriers that hide the witch from the sight of regular humans. When a witch is defeated, the Grief Seed that is left behind can be used to cleanse and restore magical power to a Soul Gem. Thus, they are highly valued by magical girls. Kyubey is shown to consume used Grief Seeds. Soul Gems may themselves become Grief Seeds if the magical girl reaches her limit--emotionally or simply by overexertion of her powers--and is unable to purify her Soul Gem.
- In Kazumi Magica, the evil magical girl Yūri uses imitation Grief Seeds known as "Evil Nuts" (イーブルナッツĪburu Nattsu) which transform humans into pseudo-witches, carrying out their dark desires.