Found in South Dakota

Not too sure what kind of rock this is, very interesting looking. Anyone have an ideas. Was found north of the badlands, South of Rapid City.


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  • $\begingroup$ Please indicate hardness of both components of the rock, whether either component effervesces in acid, exactly where it was found - i.e. host rock formation, density, and, if possible, provide a close up photograph. $\endgroup$ – Gordon Stanger Sep 18 '16 at 3:24
  • 2
    $\begingroup$ Read this: A guide for asking “Identify this rock” questions $\endgroup$ – Jan Doggen Sep 18 '16 at 19:19

You have a rock called conglomerate. This rock is basically a bunch of other rock fragments that were transported in some kind of stream or river, became rounded, and then got buried under other layers of sediment. This burial caused the individual pieces of rock to stick together to form one hard rock.

A quick look at a geological map of South Dakota shows that the area you described has limestone, sandstone, shale, and other similar sedimentary rocks. Looking at your picture - this seems very reasonable. I have a strong feeling that the white pieces are limestone, the yellow pieces are sandstone, and they grey stuff is some kind of hardened clay that cements everything together. To confirm this, do the following:

  1. Scratch it with a knife. The yellow sandstone will resist scratching. The grey clay and white limestone will not.
  2. Put some white vinegar on it. The white will fizz, the clay might fizz and the sandstone will not fizz or fizz just a tiny little bit.
  • $\begingroup$ Went to a local rock shop and what I found is a Bubblegum Agate! $\endgroup$ – user6612 Sep 18 '16 at 20:00
  • $\begingroup$ I agree with Michael, it seems to be a conglomerate of creamy coloured pebbles of country rock in a white matrix - probably calcite or aragonite. That would fit in with the geology south of Rapid City. Rockhounds often refer to rocks like this as 'puddingstone', but that isn't a recognized geological term. As to 'bubblegum agate', if it is genuine agate then the agate part of the rock will be very hard, and capable of scratching window glass. But it doesn't look quite right to be true agate. $\endgroup$ – Gordon Stanger Sep 22 '16 at 2:58

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