I found this rock near my home in the high desert of south central Utah (near Capitol Reef National Park). The sides are symmetrical and smooth -- shaped somewhat like a canoe. It is 6 1/2" long; 1 1/2" at widest point. The top is sheared off and sparkles in the light. It is embedded with flakes of a shiny, black glasslike material, possibly obsidian. The Fremont culture occupied the area and left an abundance of tools, points, and chip rock, many of which were sourced from elsewhere; most likely the result of trading. A neighbor and I came up with an uninformed theory. Would appreciate any informed observations. enter image description here

  • $\begingroup$ Do you have more photo's, preferably out of direct sunlight? This one is oversaturated. And any more information mentioned here? $\endgroup$ – Jan Doggen Sep 21 '17 at 17:48
  • $\begingroup$ I don't think the black stuff is obsidian. It definitely looks like crystals, possibly phenocrysts of amphibole or pyroxene. $\endgroup$ – Gimelist Sep 22 '17 at 10:32
  • $\begingroup$ the shape could be due to wind based weathering, it often produces smooth sided angular ventifacts. researchgate.net/figure/… $\endgroup$ – John Sep 25 '17 at 4:05

Black elongated crystals are mostly hornblendes. The rock itself is (probably) horblende andersite or hornblende dacite. Both are intermediate volcanic rocks.


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