A [[gem|gem]] birthed from a contract with [[Kyubey]]. Proof that one is a [[magical_girl|Puella Magi]]; it is the source of the girls' magical power.

Episode 6 spoilers: 
[spoiler]As titled, it is the the container of their soul.[/spoiler]
Episode 7 spoilers: 
[spoiler]Stimulating the jewel gives various sensations to its owner.[/spoiler]
Episode 8 spoilers:
[spoiler]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.[/spoiler]

Pixiv has a tag for images about [spoiler]the wrongful use of the stimulatory properties of the gem; "[b]ジェム姦 (Gem Rape)[/b]"[/spoiler]
The Pixpedia article is: "ジェム姦":http://dic.pixiv.net/a/%E3%82%B8%E3%82%A7%E3%83%A0%E5%A7%A6 
If you want to search for these images on Danbooru, try searching for {{soul_gem uncommon_stimulation}}.

h4.See also

* [[Mahou Shoujo Madoka Magica]]
ソウルジェム ジェム姦