Water content in stone
Never underestimate the amount of water that can be held in a piece of stone. When we carve our stone, we do it wet. This makes the stone easier to cut; it keeps the tools cool, and it suppresses the dust. It does mean however, that your stone fireplace is considerably heavier when wet than when it finally dries out. Not only that, but on a thick fireplace head section, the water can take a very long time to dry out. If you put the stone fireplace straight into a heated house, then the drying is accelerated, but in the case of a semi completed building project, (with no heating) it can take several weeks for the stone to be completely dry.
This is not in any way a problem, apart from the extra weight when moving the stone, and the fact that ideally the stone is best pointed when it is completely dry. This does not matter, because a fireplace is perfectly safe and secure (and still looks very impressive) even without the final pointing.
I recently had a stone mullion that needed a small repair. I could not repair it while it was wet, and it was freshly cut, and so was very wet! I decided to sit it on the storage heater in our small office over night. What a shock I had in the morning, as all the windows and the glass in the door were running with water. There was so much humidity put into the room just out of one relatively small piece of stone.