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I have found several grey rocks like this one on a beach along the ocean and I don’t know what they are. This is the only black one I’ve ever seen. Anyone know what it is?
enter image description here

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    $\begingroup$ density and hardness are important. It could be just weathered asphalt. $\endgroup$ – John Jul 22 '18 at 6:20
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    $\begingroup$ Sylvie, where do you found it? wich ocean? please provide precise location if possible. That help us to give you a better answer. Also provide a scale, we don't know if that piece is 2cm or 50cm. Anyhow it looks to me like a volcanic piroclast. $\endgroup$ – Universal_learner Jul 22 '18 at 6:57
  • $\begingroup$ If so, as said, density would be helpful. Volcanic piroclasts use to be very lightweight because they have a lot of holes. Sometimes are extremely ligthweigth in relation with his volume. $\endgroup$ – Universal_learner Jul 22 '18 at 9:09
  • $\begingroup$ Does it float in water? $\endgroup$ – Spencer Jul 25 '18 at 1:51
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It looks a lot like basalt. Basalt comes in several shades of grey and black.

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