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enter image description here

Could use help identifying this rock. It's face has some variation in color which ranges from cream to caramel. (Some variation could be from different exposure to the weather.) The rock face is satiny smooth, a contrast to the limestone that it seems to be found with. It reminds me of arrowheads I used to find in North Carolina .

There are a limited number of rock specimens at the base of a limestone outcropping. The chunks of varying sizes are uniformly about two inches in height, as if they were part of a vein with other material above and below. The "top" and "bottom" of the rock is coarse like sandpaper and there are nodules and circular pits more than an inch in diameter.

The rock is dense and heavy for its size. It readily cleaves along vertical cracks, but not so much trying cleave it perpendicular.

Describe where you found it

I found the rock(s) on the surface of the ground in a wooded area near the top of a small hill (900 feet above sea level) in Davidson County, Tennessee not far from the Williamson county line. The area has numerous limestone outcroppings, much of the limestone is embedded with brachiopods, bryozoans, cephalopods, crinoids and a variety of corals.

The pieces of this rock are scattered in a forested area that is 100 by 200 feet below a limestone outcropping.

Test its hardness

The rock is hard and dense. A brass key leaves brass on the rock. So does a stainless steel knife and the base of a cocktail glass.

I have a large piece of smoky quartz that etched the faces of the rock leaving rock dust behind when rubbed with the quartz. The quartz was undamaged

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closed as off-topic by user12525, daniel.neumann, trond hansen, Semidiurnal Simon, Jan Doggen Aug 29 at 7:05

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    $\begingroup$ Thank you for including sufficient information! Just one detail: the numbers on the scale in the first photo, can you confirm those are inches? $\endgroup$ – gerrit Jun 16 '17 at 9:47
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    $\begingroup$ If you have any acid, try putting some on a fresh surface and see if it fizzes. That would be quite useful to know. But aside from that congratulations on asking an excellent rock identification question. We don't see many of those. $\endgroup$ – bon Jun 16 '17 at 12:41
  • $\begingroup$ Yes. The ruler is inches. $\endgroup$ – Jeff Jun 16 '17 at 18:48
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    $\begingroup$ The only acid I have available is vinegar 5 percent. No impact other than to make it smell like a pickle $\endgroup$ – Jeff Jun 16 '17 at 18:49
  • $\begingroup$ Really, gerritt....do you see many centimeter scales with 16 ticks between the numbers? $\endgroup$ – Knob Scratcher Jun 18 '17 at 5:29
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Your rock is most probably dolomite (or dolostone among some circles). Here's why:

  1. It looks like dolomite. Forgive me I can't be more scientific about it, but it just is.
  2. It comes from an area with abundant limestones. Dolomite is a form of carbonate rock, derived from limestone via introduction of magnesium-bearing fluids.
  3. At first I thought it can be a quartzite, but since your quartz was undamaged it's probably not.
  4. It's harder than limestone, because your brass key didn't damage it. Dolomite is harder than calcite, the main component of limestone.
  5. It has those small holes and cavities, consistent with the rock being a carbonate rock.
  6. This geological map of Tennessee shows that the Davidson–Williamson country area comprises sedimentary rocks, including dolomite.
  7. Limestone fizzes when in contact with vinegar. Dolomite does not. One way to identify dolomite is "looks like limestone, but doesn't fizz".
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  • $\begingroup$ I agree with Michael, your rock is most likely dolomite. $\endgroup$ – Gary Kindel Jun 17 '17 at 20:09
  • $\begingroup$ Dolomite will fizz if you powder it first. This test can be tricky since the powder sometimes looks like it is fizzing a little just because air escapes as it gets wet. If it was dolomite the knife should scratch it. Some of the Appalachian sandstones have enough porosity to be oil or gas reservoirs and are often form the ridges. $\endgroup$ – haresfur Jun 19 '17 at 2:17
  • $\begingroup$ @haresfur I never trust the knife test, when taken by inexperienced people. There are so many types of knives of varying hardness. Sometimes you really need to dig into the rock with the knife to scratch it, particularly with well cemented dolomites (such as this one). The porosity in sandstone is between individual grains usually, not in the form of large holes as here. $\endgroup$ – Gimelist Jun 19 '17 at 2:56
  • $\begingroup$ Lots of dolomitic units at that zone mrdata.usgs.gov/geology/state/fips-unit.php?code=f47037 $\endgroup$ – user12525 Apr 7 at 9:54

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