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Do you have a piece of white, ceramic tile?? If you do, rub the rock on the tile and see what colour the streak is. If it's reddish brown, it'll be hematite (a hematite concretion). If it's dark grey-brown to black, it'll be magnetite (but since it's not magnetic, that rules this out). If it streaks either yellow or a brownish yellow colour, it's either limonite or goerthite. It's most likely, seeing it was found in granitic rocks/soils, a weathering product known as a gossan. Was there more of this rock lying about the place??. If so, you may have stumbled across a mineral deposit...these iron rich gossans usually form as weathering caps to mineral deposits anywhere there is a lot of iron in the mineralisation, especially in granites and iron rich metamorphic rocks.
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