As far as I can judge from the picture, it is most probably slagstone, Swedish blue stone.
Slag is a byproduct from ore smelting. In old foundries, the slag was molded and used as a constructing material for houses and roads. It is actually similar to obsidian, but is usually rather brittle. I believe that the hardness could be around 5-5.5, depending on chemical composition and weathering. The technique was introduced from England and Germany and became very popular and refined during eighteen and nineteen centuries.
It is known as slaggsten, bergslagssten, sintersten or sinnersten in Swedish language.
Bergslagen is a historical mining region and sinter has the same etymology as cinder. Sorry for links in Swedish.
Also today, foundry sands and slags are investigated and used for road constructions. There have been worries about contamination risk. See this Finish study and IRC webpage.