My advice is to do a thorough cleaning of the concrete before you start. You can accomplish this using a pressure washer equipped with a 15-degree tip at the end of the wand. Cleaning the concrete allows you to see exactly what you’ll be matching.
After your clean concrete is dry, get on your knees and really look at it. Odds are you’ll be amazed. You’re going to see the individual pieces of sand and tiny stones that were used to make the artificial rock. Your first task is to go to local gravel pits and find sand that looks just like what you see.