A gem birthed from a contract with Kyubey. Proof that one is a Puella Magi; it is the source of the girls' magical power.
Episode 6 spoilers: As titled, it is the the container of their soul. Episode 7 spoilers: Stimulating the jewel gives various sensations to its owner. Episode 8 spoilers: If a soul gem darkens completely through magic use or negative emotions without regular cleansing through Grief Seeds, it becomes a Grief Seed itself, and its owner transforms into a witch.
Pixiv has a tag for images about the wrongful use of the stimulatory properties of the gem; "ジェム姦 (Gem Rape)" The Pixpedia article is: ジェム姦 If you want to search for these images on Danbooru, try searching for soul_gem uncommon_stimulation.
