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It looks like a grindstone. The two halves would be different fineness. Your friend probably had this in his cooking kit to sharpen knives. This doesn't look like it has anything to do with star...
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#1: Attribution notice removed
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System
·
2020-06-03T15:54:26Z (over 4 years ago)
Source: https://outdoors.stackexchange.com/a/19743 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/