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That appears to be a natural, but highly polished, piece of chiastolite. It’s a a variety of the mineral andalusite which is characterised by distinctive cross-shaped black inclusions of graphite. The unusual pattern has caused it to be used as a semi-precious stone and it has been used as an amulet for centuries.
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There are large deposits of it around Georgetown in California but If you go into any new age rock & gem shop, you’ll see small highly polished pebbles that have come out of a rock tumbler being sold as ‘mystical’ items. Most of what I see in UK shops has come from Spain. I don’t know how you could distinguish modern from ancient, but to me that looks like it has come out of a rock tumbler and been lost in recent times.