We 
Hydroponics
...This growing medium has been in use quite early with great success. The popular one is the gravel-based ebb & flow system.
Gravel
As fragmented media from hard-wearing rocks like sandstone, limestone, or basalt., spaces between each particle are quite large. This helps give a plentiful supply of air
to the roots. But at the same time, that means the medium doesn't hold water well,
which can cause plants' roots to dry out quickly.
Because gravel is rocks, its weight makes a gravel based system, especially large one hard to carry. But that comes with an advantage that is durable and can be reusable.As long as you wash and sterilize it, you can start using for first time growing or reusing.
and sterilize it, you can start using for first time growing or reusing.
Just like sand, this is a good cheap starter medium for soilless growing, then people can move on to other growing materials.
Benefit 
Cheap (or free) 
Downsides 
Poor water retention, not suitable for heavy plant roots. Heavy
