Can anyone help me identify these rocks? Found these on the beach. They are VERY heavy but not magnetic. Most have a circular shape, about 1-2 inches in diameter. A few have a flat bottom. enter image description hereAny guesses?


closed as off-topic by Spencer, arkaia, Gimelist, Fred, Peter Jansson Jul 20 at 11:03

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  • $\begingroup$ A streak test would help solve it. Scratch it on the bottom side (the inside) of your toilet tank lid, which is made of unpolished porcelain. What color does it leave behind? If it's reddish brown, it's manganese. If it's dark black, it's marcasite. $\endgroup$ – Rokman Jul 22 at 9:33

I think they are most likely manganese nodules, which are formed on the ocean bed over the course of millions of years by precipitation from metal rich currents which have a volcanic origin. Although manganese is one of the commonest constituents, they also contain a great variety of other metals including iron, copper and cobalt. They often contain tiny amounts of sand grains. Most of these nodules are found in deep ocean, where in some places they lie on the ocean bed in vast quantities, but some are also found in relatively shallow seas like the Baltic. In size they range from the size of a pea to the size of a complete coconut including the husk. Yours are typical in shape and colour, though a bit on the small side. In bulk, by the thousands of tons, they have great economic potential, but the greatest deposits are in deep ocean, making them difficult to retrieve.


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