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)"
Pixiv tag search is here. -> ジェム姦
The Pixpedia article is here. -> ジェム姦
If you want to search these images on danbooru, try searching 'soul_gem' and 'pool:1622'(Uncommon Stimulation) -> this:'soul_gem pool:1622'
Read the full article.