Stone holds heat the way nothing else does. A cast tub gives its warmth back to the room in minutes. A monolith gives it back to you, slowly, for as long as you stay — basalt and granite most of all, dense enough that the bath is still warm when the water stills.
Weight is the other half of the same fact. Empty weights in the collection run from roughly 300 kilograms to 1,315 — and every one of them is confirmed to you in writing, with footprint and load-spread figures, before any balance is due. On a slab this is rarely a question; above the ground storey it is a conversation with your engineer, and we would far rather you checked than guessed.
This is what it means to bathe in stone: mass, heat, and a material that was already old before the room existed.