The Piece of Stone can be found on top of the chimney at Edison's Workshop, 1877 AD.
It is part of a larger decoration known as the Sun Stone in the Aztec time period (1519 AD), which is missing a piece. The Sun Stone is also known as the Aztec calendar stone. When you return it to the Aztecs (while also avoiding that guy with the club), their time period will be restored.
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