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A few years ago I found this unusual looking rock while walking around the beach of Cape Code, MA, USA. My guess is that it is sandstone, but I didn't see any others like it on my stay. It is has smoothed out edges, and is about 2 inches in height.

Update: Lightness - It seems lighter than rocks of similar size

Hardness: 3 - A dime scratched it a bit, but not my fingernail

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closed as off-topic by Leukocyte, trond hansen, daniel.neumann, Semidiurnal Simon, BillDOe Aug 29 at 20:13

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  • $\begingroup$ I would say yes, it looks like one. $\endgroup$ – Gimelist Jan 6 '16 at 8:06
  • $\begingroup$ Could you provide more description? See this guide, but in particular, how heavy is it (relative to other similar sized stones), how hard is it, and what is the texture like? $\endgroup$ – naught101 Mar 7 '16 at 22:24
  • $\begingroup$ After the edit: I still think it's a sandstone. Sandstones usually feel a bit lighter than rocks of similar size because they're full of porosity. About the hardness, I'm not sure you actually scratched it. The scratch test is useful when you have big minerals inside. This is an aggregate of sand-sized quartz crystals, and the dime possibly removed a grain or two to make it seem like it was scratched. $\endgroup$ – Gimelist Jan 19 '17 at 0:13
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Argillaceous Siltstone. Sedimentary rock made from silt with a grain size bigger than that of clay but smaller than sand.Siltstones often show bedding planes and even ripples, where successive layers of silt have been deposited.

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This looks like gneiss. Sandstone is a sedimentary rock, meaning it formed from layers of sedimentary particles that are sand sized: 1/16 to 2 millimeters in size. These layers were pressed together by the weight of the layers above them. Sandstone can easily be broken apart. Gneiss is a metamorphic rock. Usually make by pressure and heat from one direction of the rock. Sandstone can be converted in to gneiss from this pressure and heat. It is harder than sandstone and considered to be high grade metamorphic rock. This grade means that it was under the same pressure and heat for a longer time than medium grade and low grade. This ultimately means that it is stronger and does not break easily. Your image showed small chips in it. This is why I think it is gneiss rather than sandstone. Your stone is roughly polished from the sea. The picture on the left is sandstone and the picture on the right is gneiss

sandstone gneiss

Here is a chart that might clear up any confusion and yes it would be nice if we had the rock to identify more precisely metamorphic folliated/non-folliated chart

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    $\begingroup$ There is insufficient competency for this to be gneiss. Look at the section that has broken off, an impact to gneiss that would be hard enough to make that divot would cause it to fracture conchoidally. $\endgroup$ – Ben MS Jan 9 '16 at 0:40
  • $\begingroup$ I agree with @BenMS - I think this is sandstone and not gneiss. Too bad we can't hold it in our hand - most of us would be able to tell in a second. $\endgroup$ – Gimelist Jan 10 '16 at 7:24

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