Metallic luster Grey crystals, appear black in photo, grey streak With quartz Non-magnetic Softer than stainless steel blade, harder than fluorite =>hardness ~5 Found in California on the eastern flank of the Sierras in granitic terrain photo of sample 1 photo of sample


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  • $\begingroup$ Is the rule in inches? What's its density?Also to search on USGS you could provide a link to the Sierras eastern flank with the precise coordinates on Google Maps.. $\endgroup$ – Leukocyte Jul 16 at 17:39
  • $\begingroup$ Hardness of what? I see several minerals on your rock $\endgroup$ – Leukocyte Jul 16 at 18:52
  • $\begingroup$ @Universal_learner how the ruler is divided will tell you it is in inches, 16 units per mark, means not metric. $\endgroup$ – John Aug 18 at 5:59

Your rock is a pegmatite, which is an igneous rock similar to granite and distinguished by its large crystals. The usual cause of large crystals in igneous rocks is slow rate of cooling, but in the case of pegmatite the cause is believed to be superheated water in the magma as it cooled, transporting mineral ions to form the crystals. The fact that it was found in association with granite supports that conclusion. Like granite and marble, the composition of pegmatite varies a little, so they don't all look the same, but yours is fairly typical. Pegmatites sometimes have large crystals of semi-precious stones in them, so it might be worth looking around if you ever go back to the place where you found this one.

  • $\begingroup$ Depending on which reference one chooses the crystal size in a pegmatite must be larger than either 1 cm. or 1 inch. In this case the crystals are smaller. Also, the pegmatite classification is very general and I am interested in which mineral the dark crystals might be. $\endgroup$ – rock_fan Jul 17 at 18:54
  • $\begingroup$ The black crystals are probably hornblende, which is a variety of the silicate mineral amphibole. $\endgroup$ – Michael Walsby Jul 17 at 22:27
  • $\begingroup$ Very well, that fits. $\endgroup$ – rock_fan Jul 20 at 1:57

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