What the heat does
Cold muscle resists. Before working a tense area with pressure, a hand spends several minutes just preparing the ground, and on some backs that preparation eats half the session.
A warmed stone does that work on its own, and it does it while I am already somewhere else. The heat goes in slowly, the muscle gives before it otherwise would, and the pressure that follows does not have to be as firm to reach deep. That is why this is the longest session: ninety minutes, because heat needs time to be worth it.
I do not use stones for any special mineral property. What works is warmth and weight, applied slowly.