How thick are they? Don't they need to be backed by an even support in order not to crack and then get pushed around? On a side note I need a new fireplace hearth...J/K I'm going with a single blue stone slab.
1-2" thick. I wouldn't make a bridge out of them, but i'm willing to bet they get pressed in nicely, hence fully supported.
Complete with trip hazards from being uneven.