I need a little help identifying this rock. It was found somewhere in the Sahara Desert. It is very hard and leaves a beige streak on a steel file (might be the concretized dirt covering it). The polished window shows little black specks scattered around. There are also some tiny shiny specks which the camera doesn't capture. No discernable layering. It is non-magnetic. Measures around 3cm long (1 1/4").

I've never seen a rock like this before, would appreciate some help identifying it!

Mystery rock Mystery rock enter image description here


closed as off-topic by Jan Doggen, user12525, Gimelist, Peter Jansson, Fred Aug 26 at 17:34

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  • $\begingroup$ Did you find it in a pile of other such pieces? $\endgroup$ – Spencer Aug 18 '18 at 13:27
  • $\begingroup$ I wasn't the one who found it, but as far as I know it was picked up individually. It wasn't part of a pile. $\endgroup$ – Liang Geng Jie Aug 18 '18 at 14:38
  • $\begingroup$ I can't comment on the material but the flat pitted sides are common in wind eroded ventifacts. $\endgroup$ – John Aug 18 '18 at 18:55
  • $\begingroup$ crack it open so you can see a fresh surface $\endgroup$ – haresfur Nov 16 '18 at 23:01
  • $\begingroup$ I can't answer this, but I wanted to say well done for actually asking a rock-indentification question with all of the info that's requested in the relevant meta article! $\endgroup$ – Semidiurnal Simon Nov 18 '18 at 15:06

Given its hardness, red-beige matrix color and black inclusions (obsidian?), I tentatively conclude that this is some form of rhyolite.

  • 1
    $\begingroup$ Obsidian does not occur as "inclusions". Obsidian is a rock which is entirely or mostly composed of glass. $\endgroup$ – Gimelist Aug 19 '18 at 7:48
  • $\begingroup$ In that case, perhaps the black specks are biotite $\endgroup$ – Liang Geng Jie Aug 20 '18 at 2:52

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