What if those two were perceptive.

This is a Soul Gem.

This jewel is produced when you make a contract with Kyubey.

"Soul Gem"...so basically, a jewel made of our vital force. Sayaka is translating "Soul Gem" in Japanese, since in the anime it's used in English.

So that's Mami-san's soul, isn't it?

How perceptive of you, Sayaka-chan!

...huh?

Yes, that's right, Sayaka.

...huh?

Kyubey?

It's obvious that middle schoolers, nowadays, can understand at least this much of English.

Am I right, Mami?

Ah...uh...yeah...