I found this grenish rock at a beach in Northwest Ireland called Inishcrone.

  • It measures 45mmx35mmx22mm.
  • It weighs 57 grams and has a density of 2.90.
  • It is opaque, does not attract a magnet.
  • It has a hardness of 6.

2 3 enter image description here

I found it among other rocks like Jasper and Quartz

enter image description here

  • $\begingroup$ Interesting rock, looks like massive epidote but the density is too low. $\endgroup$ – Friddy May 9 at 20:28
  • $\begingroup$ Maybe serpentinite but I don't think it will be possible to positively identify from a photo. Try googling to see if serpentinite occurs in the area $\endgroup$ – haresfur May 9 at 22:47
  • $\begingroup$ Yes there is Serpentine in the area but its density is greater than Serpentine and harder than any Serpentine I have found here in Ireland. Thank you haresfur $\endgroup$ – Dave May 9 at 22:54
  • $\begingroup$ Just wondering if it could be an opaque type of Nephrite? $\endgroup$ – Dave May 9 at 22:57
  • $\begingroup$ Thank you for your comment Friddy $\endgroup$ – Dave May 10 at 8:55

My guess is your rock contains epidote that gives it the green color, but also other minerals as quartz, feldspars, amphiboles and/or carbonates.

You can find likely rounded rocks containing massive epidote at this webpage.

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Epidote in unakite: Tumbled stones made from unakite, an igneous rock composed mainly of green epidote, pink orthoclase feldspar, and quartz. This unakite was mined in South Africa. Source: geology.com

Epidote's density is a bit higgger than the value you provided: $3,4 g/cm^3$. Your rock does not appear to have pink feldspar but it could contain quartz, feldspar or carbonates. As density of those minerals rounds $2,7 g/cm^3$, the resulting density for the rock would match with your measure.

The mindat description for Inishcrone beach says there is a dolerite dyke in contact with the carboniferous limestones of the area. Epidote appears at dolerite dykes.

My guess is it comes from the dyke, or from a surrounding skarn system.

A skarn is an hydrothermal alteration of carbonates, that would also match with the geology of the zone, described in this old paper. This alteration appears when fluids of igneous origin (maybe associated with the dyke) increase the temperature of the limestones they cross.

There is also a gabbro intrusion at the west side of the bay. It is unlikely to find epidote on igneous bodies but the rounded rock could have been transported from the intrusion. My guess is it is too much faraway to be the origin and epidote is rare on gabbros as said.

geological map of Irishcrone Geology of Irishcrone site, Ireland. Bedrock Geology 100K. Geological Survey Ireland.

To determine the origin you would need to identify the minerals that go along with the epidote. A fresh-cut and a picture with a magnifying glass could clarify it.

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    $\begingroup$ That is a very comprehensive answer that makes complete sense, thank you kindly Universal_learner $\endgroup$ – Dave May 16 at 11:51
  • $\begingroup$ @Dave. I need to reedit my answer as a third environment for epidote is skarn, alteration of limestones that would also match with the environment. $\endgroup$ – Universal_learner May 16 at 12:08
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    $\begingroup$ Great stuff, you have given a very indepth understanding of the environment where the rock possibly originated from a Skarn or Dyke in the given area, this is very helpful. I don't think I'm going to cut it though, like it too much, there is a small area on the stone where I'm pretty sure Quartz is visible. Thank you Universal_learner $\endgroup$ – Dave May 16 at 12:32
  • $\begingroup$ @Dave you are welcome and thanks for accepting my answer. Te problem for serpentine is where does the Mg come, while epidote is common on Ca rich systems. I guess it is an altered limestone on a skarn environment. The fluids are sometimes rich in Si and Quartz can be developed on this replacement too. $\endgroup$ – Universal_learner May 16 at 19:41

A small fracture on the surface of the stone causes the outbreak of the white section inside it, which, due to its density, can be considered as an agate. Due to the presence of Jasper and quartz parts, this is reinforced.

  • $\begingroup$ Thank you for your comment Kia $\endgroup$ – Dave May 21 at 7:21

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