Any inert, durable granular material of the correct effective size and uniformity coefficient can be used for slow sand filtration (Buzunis, 1995 [ref.01]Ref.01: Buzunis, B.J. (1995) Intermittently Operated Slow Sand Filtration: A New Water Treatment Process. MSc Thesis, University of Calgary, Canada.). Mostly, this takes the form of sand, which is readily available in most environments, but there are other alternatives where sand is not readily available.
The links on the left provide information on the filter media for sand filtration. Firstly there is an explanation of why the size of the filter media is important at all. The terms effective size and uniformity coefficient are explained, alongside a worked example of completing your own sieve analysis to determine what kind of sand you have in your area. Following that there is a section on what specific grain size is recommended for sand filtration in general, and the effect of different sand sizes and sand bed depths on parameters such as pathogen removal and colour improvement. There are various methods of enhancing the performance of the sand media in terms of water quality, flow rates and corrosion control, and alternatives to sand as a filter media. Lastly there is an explanation of the role gravle plays in the sand filter.
Ref.01: Buzunis, B.J. (1995) Intermittently Operated Slow Sand Filtration: A New Water Treatment Process. MSc Thesis, University of Calgary, Canada.