This appears to be limestone, which would not be precast. You have movement that has occurred between the pieces and it has opened up a masonry joint. It needs to be repointed. If they don't want to put the money into that you can use an exterior grade clear
caulk like NP-1 but it will not make it look any better, it will only seal it from water intrusion. Don't try to caulk with "limestone colored caulk" it will look terrible. To fix it requires a skilled mason to cut out and replace the joint, once they have
been sure there is no current movement. Concerning the staining, it depends on the age of the structure and the likelihood you might be dealing with carbon stains from the coal burning days. Prosoco is the best quality exterior stone and masonry vendor I
have come across. Their tech line is the best in the business. This would benefit from a natural stone sealant once all the work is done to help shed water and keep the absorbent stone from easily resoiling.
Joyce Harris
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