Pros may have better advice, but I would use modified thinset mortar like is used for tile underneath the stones. HD or any others will carry it in their tile section, not with the cement.

Matching the mortar between the stones will be more of a challenge if you chip that out. The original was probably mixed on site. Could have been grey or white cement plus sand and maybe some tint. Maybe try buying sanded grout from the tile section again that matches the existing mortar.