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This cladding is called stack stone and was very expensive in its day. But the glue can fail underneath, causing the stone to fall off.
Normally I would say to glue the stone back on, but it will fail again as the under layer has perished.
So the next step would be to scrape the glue off.
Try tapping them with a hammer and a broad flat screwdriver or a brickies chisel.
Once you scrape off the glue you will be able to see what they are mounted on, such as concrete or brick. - Jewelleryrescue
It's great that you've received excellent advice from @Jewelleryrescue. I agree that the stone will need to be removed as the adhesive has failed.
First, take some photos of the stone face remaining. You can use this as a guide when putting it back together. As you remove each piece, I suggest putting each section in its own container so that you won't get confused when it comes time to re-assemble. Make sure to take photos of all the sides so that you will have a guide.
I then suggest grinding the old adhesive off the wall and the back of the stone pieces with a grinder.
Once all the old glue is removed, I recommend using Davco 20kg SMP Evo Tile Adhesive as it is compatible with stone, marble and porcelain tiles. Plus, it can be used outdoors. - EricL
