I found this on the beach in Rhode Island. Although similar at first glance to coal, it's more gold-ish/bronze iridescent and very hard. 2" wide. It would not flake or scratch with a piece of metal.

It has no smell at all. It's very "layered" looking but not chippy like mica. It's very hard and doesn't leave marks like a graphite or softer coal would. The iridescent finish looks a bit like peacock anthracite coal except it's more bronze than anything. The back of it is not as iridescent. You can see some pits where other harder material might have been when it formed. I imagine it spent a lot of time on the beach getting smoothed. I read that RI used to have anthracite mines. This looked very different than other coal I've seen on the beach which is why I kept it.

I washed it and also noticed that the bottom has a more blue black layer iridescence on part of it then it transitions to bronze tones on the side and top.

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closed as off-topic by Leukocyte, trond hansen, Gimelist, uhoh, Fred Aug 27 at 16:24

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  • $\begingroup$ Could it be coal with pyrite fossils? $\endgroup$ – The Garage Chemist Jun 25 '17 at 14:30
  • $\begingroup$ Black shale with pyrite perhaps? $\endgroup$ – Elysa Jun 25 '17 at 16:19

Your rock is mostly anthracite coal. You best diagnostic test is density. Coal even anthracite is generally the least dense rock that has no visible porosity.
As for the colors, coal typically shows a large range of surface sheen colors but due a number of impurities like complex organic compounds and metal sulfides.

  • It does not smell like sulfur or organic material.
  • It won't leave a black or grey mark.
  • Hardness is: >2.5 to ~3 Mohs scale
  • Relative density of 1.3–1.4 Compare this to shale:2.4 - 2.8 or slate:2.7 - 2.8
  • No visible pores

Reference: Nice article on mineral and rock densities.


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