Gravel is made of a variety of rock fragments naturally formed through weather beating and forces of water, which is why riverbeds are often lined with it. The supply of gravel produced in nature is insufficient for consumer demand, so gravel is mined and transported using fossil fuel draining processes. It's costly both on the pocketbook and the environment to mine new gravel and since it's such a robust material, many recyclers are keen to reclaim used gravel for cleaning, sorting, and stockpiling for use on future building projects alone or for concrete aggregate.