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I found this rock on the beach on Long Island, NY. It's very light. At first, I thought it was pumice. But it doesn't have many holes. It has a lot of beautiful cracks through which a black shine comes through. There are two spots, in particular (tried to photograph them here) that have a bluish shine to them. A knife scratches it if I push hard. A penny does not, so I guess it's a 5.5 or perhaps a bit higher (I had to push hard). The luster of the shiny parts is sort of...like a pencil tip? I don't know how to describe it.

Any ideas? Did I just pick up a chunk of road tar that made its way into the ocean? Or is it a black opal? 😜 I'm just curious--and so are my kids!

Thanks in advance!

https://i.stack.imgur.com/tGUcl.jpg

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  • $\begingroup$ At first it looked like a weathered sample of hematite, but hematite is very heavy. Jet is also a possibility as Ash suggested. Overall to me the most likely would be either a high grade of Bituminous Coal or Anthracite. Both these coals can have harder sections with metallic like luster. Both are lighter than most other typical rocks and both would weather similar to the sample you have shown. Unfortunately neither of them are are hard as copper, max of 3 on Mohs scale. I would suggest doing the hardness test again and see if the penny marks the rock or the rock marks the penny. $\endgroup$ – Friddy Oct 30 '17 at 21:10
  • $\begingroup$ Petroleum Coke? $\endgroup$ – Keith McClary Jan 29 '18 at 4:53
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looks like tar, It could come from any number of sources. oil spills, roads, old piers, ect. The light weight just helps confirms it. Depending on what is mixed into the tar getting a correct hardness may be tricky.

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I think it has to be a lump of bitumen (road tar) but the hardness can't be right. Does it start to dissolve in white spirit or in any type of solvent? And how does it react to heating, is it getting soft?

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Could be Jet which is a mineralised form of coal, it also reminds me of several pieces of salt mineralised charcoal I have that I found on beaches in New Zealand.

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That is DEFINITELY a lump of coal!

Merry Christmas!

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