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
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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.