Grout, mortar, or something else?


Jan 18, 2017
New Orleans, Louisiana
I hope I’m posting in the right place. We have flagstone coping and some of the material between the stones is cracked. I chiseled out a line assuming it was grout but it seems much harder than any grout I’ve used (for tiles). If possible would someone be able to attempt to help me identify the material? Pics included.3FC2DCE5-C038-4B2B-A378-561DECC8F50A.jpeg91170D00-4882-4B31-8DA8-588B314281B6.jpeg7A13A0FF-FD60-49DB-BF60-A68181F3BC56.jpeg