I think this all depends on which country you are in. In the UK granular fill is generally any material with less than 15% CLAY and SILT fraction, i.e. less than 15% passing the 63µm. The GRAVEL sized fraction on the other hand is a general descriptive term used for material which is finer than 63mm and retained on a 2.00mm sieve, which could include natural rounded gravels, crushed rocks or even secondary re-cycled materials such as crushed concrete. Natural gravels tend to be used as a descriptive term for a weathered rock mass which has formed a smaller fraction within the 63mm to 2.00mm size, and are generally rounded to sub-rounded pieces, with the parent rock geology falling into one of the primary rock sources, i.e. Ingeous, Metamorphic or Sedimentary.