Impact by a hard hammer depends on 60-degree shockwave propagation to detach flakes. (Like a BB hitting glass...)
The remaining "scar" is a distinctive, "shell-shaped" ("conchoidal") depression radiating from the point of impact -- that looks very much like a clam shell.
And, the detached flake has matching, distinctive "bump" on it (called the "bulb of percussion". And, you can often see/feel shockwave "ripples" on the surfaces:
They are easy to see -- and even easier to feel with the fingers...
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And, only a fast-moving hard object (hard-hammer blow) produces a conchoidal scar. No natural process has been shown to produce them.
A couple of minutes "hands-on" with those specimens would suffice to determine if they are "just rocks" or not.