Apology. I have level 15 Arcanist (ACN).
Have you completed your Conjurer level 30 quest?
Edit: Specifically, you have to have completed In Nature's Embrace and Sylph-Management (which is a level 20 MSQ).
I've completed all level 30 quest. the final one was with Sylphie. After, I returned to the guild for the final cutseen, but was not given the option for the Soul Stone quest. I'm looking at the journal and it shows I completed In Nature's Embrace. I complete Sylphies Trials at level 20.