Sanity
From that day forward, Gremory didn't need a Qi Stone as long as he was meditating in the Elf Forest, the condensed Qi was literally everywhere, and there was plenty more than what Gremory could consume in his life.
The cultivation became tougher but not harder once Gremory understood the level of concentration required. Of course, it took months for him to learn the basics of cultivating without Qi Stones, and he was not sure if he could do it outside the Elf Forest.
Usually, it takes decades if not centuries for the Qi in the environment to condense into the stones, so the stones are nothing but the concentration matter of Qi. In this forest, Qi is abundant what possibilities Gremory to cultivate without those stones.
Yet, after learning this, he was not sure if it was possible outside the forest. He never got out, afraid of leaving the safety from the forest. Not afraid for his life, but afraid for what could happen to the others.