It's important to remember how young he is. He's all of twenty one years of age, and handles himself with the composure of a seasoned veteran. Indeed, sometimes he has too much composure, in the sense that he will hang onto the ball when it would be more advisable to give it a solid boot into touch.
His ball playing abilities out of the back is what makes him such a highly sought after player. He isn't a half bad tackler either, but needs to do a lot of work on his positioning and awareness to reach the next level of his potential. That said, his confidence in possession makes him the perfect center half for Martinez's style.
But being only twenty one, he needs the guidance and leadership of Jagielka alongside him to truly show his best form at this point in his career. We've seen in abundance from last season into this one that when Stones and Jagielka partner together in central defense, Stones looks a completely different player. You start to understand why he's being valued at these astronomical sums when he plays as he does in those occasions. Swansea away from this year comes to mind, when he not only simply ghosted by Swansea's press when on the ball, but also hardly gave the attackers a sniff at goal in a performance that at times defied words.
And to be fair to Stones, the style the manager implements leaves the defense with a lot of work to do, and often not enough hands on deck to do it. I think we will see an uptick in our defensive cohesion(have already seen such over the last couple of weeks) when Coleman returns and Stones can play alongside Jagielka again.
It'll be a sad day when he leaves, because he is a truly gifted player. But the transfer fee we receive will soften the blow by a good amount, I'd imagine.