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I'm in southern California, hiking through a dry creek bed. Half of the rock is very smooth, and dark brown, the other half is grayish brown and textured. There are four holes in the rock, two of which have white rocks inside the holes. I could not make a noticeable scratch with a nail.


closed as off-topic by Leukocyte, arkaia, Fred, trond hansen, Gimelist Jul 19 at 10:46

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  • $\begingroup$ This is chert. Search "chert" in this website for plenty of answers to questions on similar rocks. $\endgroup$ – Gimelist Dec 20 '17 at 9:59
  • $\begingroup$ The fracture on the black host rock is not typical for chert. I would suggest the host rock might be an extrusive rock perhaps basalt. The holes could have been vesicles that have filled with silica through a hydro-thermal process after the host rock was originally formed. This is common in many parts of the SouthWest US. I have found similar type stones south of Medford, and in New Mexico. Both have been in old volcanic flows. $\endgroup$ – Friddy Dec 20 '17 at 18:15

Your rock might be one of the following:

  1. Piece of limestone or dolostone with with two large chert nodule 'eyes'

    • Unbroken surface of the limestone is dark due to iron leaching or desert varnish.
    • Broken surface appears to look like dark grey limestone. White grains might be fossil bits.
    • 'Eyes' white chert nodules. To Confirm: Apply HCL to the broken surface. Should react if it is limestone.
  2. Piece of slag....

    • Unbroken surface has a melted appearance.
    • Broken surface shows a weak band or rind near the surface that shows some iron color. Might be leaching or affects of heating/cooling.
    • 'Eyes' might be siliceous dross material that was attempted to be driven off by a smelting process. To Confirm: Will not react to HCL. and its density should have it be heavier than it appears.
  3. Chert - Chert can be all colors with variations and can look smooth. To Confirm: Will not react to HCL. Density should be ~2.648 g/cm3. Should feel as heavy as a similar sized quartz pebble.

  4. Basalt or similar volcanic rock.

    • Unbroken surface smooth and and broken surface showing very small grains. (This rock would have cooled rapidly. Not enough silica in it to form glass though).
    • 'Eye' could be siliceous material or zeolite mineral deposited after rock cooled. To Confirm: Will not react to HCL. Density would be higher than ~2.648 g/cm3 for quartz.

My best guess is its either slag or a dark limestone with chert nodule eyes.


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