Fossils, concretions, and other false friends
Echinoids, pyrite suns, and stones that look made but aren't.
Some natural objects imitate artifacts relentlessly. Fossil echinoids (sea urchins) look like carved domes with five-rayed stars; crinoid stems resemble beads (and were actually strung as beads in the past); belemnites pass for bullets or points; flint nodules grow into faces, tools, and animals.
Concretions — rounded mineral masses in clays — mimic cannonballs and eggs; pyrite discs radiate like tiny suns; septarian nodules crack into 'turtle shells'. None are made, though several were collected and valued by past people, which can make a natural object a genuine find in context (a fossil in a grave is an artifact of behaviour).
The test is material and structure: fossils show organic architecture through and through; concretions have concentric internal structure; tools show flake scars. When something is natural, say so cheerfully — and if it came from an archaeological context, record it anyway, because ancient people liked curiosities too.