I'm surprised SHH hasn't heard.
Mjolnir = Soul Gem. It's why Hela's death-touch power can destroy the warhammer's form, like when she kills someone. But she can't destroy their soul, she just kills them. Which is why the warhammer shatters.
It also explains why Vision, someone imbued with the Mind Gem, is *worthy of Mjolnir, which contains another Infinity Gem. I mean, if you noticed, Thor comes back all electrical and such, right? If you watch Mjolnir shatter, same power. As if he's the "hammer" now. People've said that one over and over, that the power was inside him all along. Not so sure that's exactly the case, though. I think he rediscovers the power and realizes he doesn't need it. I mean... If you've seen the Infinity War trailer... Nvm. No spoilers.
Not that big a grain of salt. Think of the MCU and its fusion of concepts within the original MU. Celestials have changed to being Collector, Grandmaster, Ego... And the Infinity Stones themselves... Loki's Scepter, Cosmic Cube, Aether, Eye of Agamotto... Those are all their own item of power in the MU, but in the MCU they've doubled as Infinity Stones.
Mjolnir is the Soul Stone. 80+% probability.