Rock materials are better than artificial products because they are weather-resistant. They would not grow faint from sun rays and also shed moisture. They will not rust with humidity and insect damage. Moreover, if rock begins to fall off the roof than it’s a warning that tar under the rock is dry and fragile. Thus it’s high time to re-roof. If there are gutters, check them. They are most likely filled with the rocks. Your roof is almost 50 years old, you should start to save money for a new roof.