After big tides on the shore of the island Helgoland collectors are able to find a special variety of flint. It is reddish coloured, coated first by a thin white and a following black layer. This specimen are highly desired by collectors and tourists (even some jewellry is produced by this material).
If the concretions were split off they are showing tiny little agate-formations, sometimes drusy megaquarz and a lot of different fossiles (e.g sponge-needles and small polypes).
More informations about the locality and the formation of this material are given at www.flintsource.net