Scale: Entire rock is 3cm in diameter.
MOH brough this little gem home from a hike one day, and it was immediately in the running for top shelfcrop, seeing as how Halloween is nearly upon us and it resembles an cheerfully smiling jack-o-lantern. It has a big grin, one partly squared off round eye, and one triangular eye. In reality, it's a dark gray, brecciated dolomite (now running out to the truck to get the HCl in order to test this diagnosis - yes!). Some breccia clasts appear to have been rotated or at least moved (the triangular eye) while others appear to be nearly in place (part, or even most, of the smile); the clasts were then cemented by a matrix of white, coarsely crystalline
This is a sedimentary rock (dolomite) that has been brecciated under unknown circumstances, given that the context (an outcrop) is missing. Consequently, I don't know what kind of breccia this is. Possibly it's a karst breccia; maybe it's a hydrothermal breccia; maybe it's a sedimentary breccia that was cemented with dolomite. Maybe it's some kind of fault or tectonic breccia. Just don't know.
Oh, and then I had to do this, thinking I saw a couple veinlet correlations...
...and then this...
...and then this. Purple outlines the breccia fragments; dark pink follows veinlets.
And here's our grinning jack-0-lantern breccia in it's current home, on a shelf, leaning against a folded towel, making it truly a shelfcrop, and a top shelfcrop, at that!
Or, Happy Nevada Day, depending on which you prefer to celebrate. :)