Given that the sides of the rocks and not just the outside surfaces are exposed, the coping stones were most likely done as exposed aggregate from day one (they precast the pieces, spray retardant on them, and then after the core has cured, wash off the surface, which exposes the aggregate). Replacements would likely have to be custom made by a concrete counter top contractor or pre-cast concrete statuary manufacturer, but there's no way that they'll be able to find the mix of rocks to make the color and texture an exact match. That'll cost you an arm and a leg to have custom done, so entire replacement may be best.