šljunak

1. (on roads, in building) gravel
   Put do vikendice je posut šljunkom. — The road to the cottage is covered with gravel.
   Kamion je doneo šljunak za betoniranje. — The truck brought gravel for the concrete work.
2. (on beaches, in rivers) pebbles; shingle
   Više volim plaže sa šljunkom nego sa peskom. — I prefer pebble beaches to sandy ones.

Usually a collective noun used in the singular: mnogo šljunka.