Me and some of my friends have found many rocks similar to these in central Europe - we believe that it is fossil-rich chert that originally comes from Scandinavia.
We find many of them in southern Moravia - there are many sandpits with quartenary sediments (mostly sands and gravel) that were deposited by river Svitava. We think that these stones must had been transported by glaciers from Scandinavia and then deposited by rivers.
I have searched for this material on the internet and could not find any informations about its formation or deposition. Do you have any informations please ? Could you provide me some links, pdf-thesis or an explanation ?