Black Stones

On a recent Sunday walk in the fields north of my home town at the Norwegian West Coast I found a hard, black pebble with some rust on.It is magnetic and turns a compass needle, and is not extremely heavy.In the vicinity I could find more of this black material, on a ground recently levelled for construction, where parts of black stone were spread among otherwise very well-known mouse-grey rock.

North of Haugesund

In hobby astronomy it is not possible to avoid the element iron - the molten iron core in our spinning earth is responsible for the magnetic field orientating the compass, and the magnetic shield around the earth protect life from harmful sun wind radiation.Also iron is the heaviest of elements synthesised from hydrogen inside stars before they die either as a cool white dwarfs (likely the fate of our sun) or as supernova.
Through the life of a star iron is synthesised in a stepwise manner in the nucleosynthesis where hydrogen atoms first fuse to heavier helium atoms which in turn merge stepwise to molecules like oxygen and silicium before finally ending up as iron before physical laws limits further processing.
Depending on the stars mass its fate will either be to graciously expand to a red giant star later slowly contracting to a cool dwarf, or if heavy enough its final days will be sudden contraction before exploding as supernova.Indeed a violent event where even heavier elements like lead and gold will form and mix with already existing molecules of silicium, iron etc. into a cloud from where other things can be born - like our solar system.
The Norwegian Geological Survey has existed for more than 150 years and now have a brilliant web site with detailed geological maps of Norway.Another service offered at the site is "ask an geologist".I mailed a photo of the pieces to NGS and asked for clues for age and how the rocks was formed (pebbles 1 & 2).The next day I had a kind reply from geologist Mr. Solli taking precaution interpreting only a picture, but quite convinced the rock is iron enriched remnants of sediments from sea bed pushed up at land 450 millions years ago, when Greenland collided with Western Norway.Huge mountain chains were then formed (the Caledonian era), now almost eroded away, but leaving the black stone.
The push-up of seabed had stopped almost where I had my walk, and the grounds just a few kilometres further to the north was dated to be 6-700 million older, from the Precambrian period.Pebble no. 3 is picked from this area.
The atmosphere in Precambria was filled with nitrogen, methane and carbon dioxide.In the sea only nitrogen fixating anaerobic cyanobacteria existed, releasing oxygen as a waste product.For a long time these first free oxygen molecules was consumed by an inorganic reaction precipitating ferrous ions dissolved in the sea, before atmospheric oxygen content became significant enough to allow more advanced forms of life.In the Caledonian era life still did not exist on land but fossils reveal octopuslike, now extinct organisms had evolved.A possibility could then be the black matter in the stones is ferrous oxide laid down on the sea bed by cyanobacteria years ago - cool!.

Much of the mapping of stellar nucleosynthesis was done by Ukrainian physicist George Gamow who also wrote easy read books on physics.His book "Birth and death of the sun" from 1940 are in the public domain at archive.org here for download in different e-book formats.