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I found these rocks at a road in Sweden. They are different from the other rocks used for the road.

The hardness is higher than 5.5 as the rocks aren't affected by a knife. the size is 2.0-2.5cm They are a bit blueish and they are striped blue as you can see on the picture. It feels like a normal rock and it has a bit of dirt i think on it.

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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.

wiki common

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.

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    $\begingroup$ It looks somewhat like obsidian too $\endgroup$ – L.B. Aug 16 '15 at 18:35
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    $\begingroup$ Yes, and as it actually is melted silicates, it also has the same amorphous shape but often with rather strong colors. However, obsidian is per definition of volcanic origin, and there is little of that in Scandinavia, but at least historically, plenty of foundries. $\endgroup$ – Tactopoda Aug 16 '15 at 18:50
  • $\begingroup$ I was going to say obsidian, but then I've read your answer. Interesting! $\endgroup$ – Gimelist Aug 16 '15 at 23:27
  • $\begingroup$ Yeah we had a old foundry in our town, so thats where it probably came from. Thanks :) $\endgroup$ – alfred Aug 22 '15 at 15:04

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