Discovering Magnetite: Black Rocks That Trigger Metal Detectors

Question

1  

2   A friend of mine recently found a number of black rocks that set off his metal detector, and attract a magnet. These are varying in size and shape, and are mostly found in/near a stream in the woods. The rocks themselves are heavier than one would expect from their size. Any idea what he's found?

Answer
The specimen as indicated by picture 2 is most likely the mineral magnetite (Fe3O4), a very common mineral, often mined as an iron ore. It will attract a magnet and is very heavy. Picture number 1 looks more like a piece of slag. Any iron smelting facilities close to the point of discovery? Of course positive mineral identification is somewhat limited from pictures and descriptions. If you want to know more, you may want to consider our free mineral identification services from the Metropolitan State University of Denver as part of our community outreach. For details please see: http://college.earthscienceeducation.net/MINPET/MINID.pdf