Army Painter Battlefields - Ash Grey
This flock looks like a rocky underground, small debris made simply as the name suggests: Ashes from a battlefield in fire.
The Battlefields series has been made to give unlimited options to the wargamer, when he/she decides on what type of bases the miniatures should have.
The specialized Battlefields is called XP, and these will add very realistic and cool effects to any base. If you want to make an army a cut above the rest - it starts with the bases.