I don't know much about building a RMH but have quite a bit of experience working with limestone in outdoor fire places. It is very common here and comes in different grades, for lack of a better word. I just go down to the creek and collect what I need and have learned from experience what is good and what isn't. Some will crack or even explode if over heated, while some holds up just fine as walls of a fire box.
Even what I consider the higher quality type would not take much heat if it less than an inch thick, I'd say it would need to be three or four inches thick. If I'm envisioning your plan correctly thin slabs would be fine as a finish on top of something else like brick but not on their own.
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I was thinking that the actual temperature at the top of the benches would be fairly low, hopefully much less than 100 degrees C. If it's going to be more than that, it's probably going to be too hot to sit on...
As I understand the question, an outdoor RMH will be made with 2 benches, and you want to finish the tops with limestone. That will work just fine, assuming you will have several inches of filler/mass underneath the slabs to absorb the heat. If properly sized, I've read that the surface temps shouldn't go over 140F/60C. Not only would that be too hot to sit on, adding something like a blanket over it to "cool it off" will trap that heat and result in even higher temps, which could cause the covering to smoulder and catch fire eventually. I've seen user reports more typically in the 80-110F/27-44C range. Every build is different of course.
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I think you would be fine as long as the limestone slabs are not directly exposed to the hottest part of the exhaust gases as they exit the combustion area. For example. Set them on top of a layer of cob. The limestone will likely only crack if it is heated unevenly.