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Also has a small inline indentation on the backside that is nowhere near as prominent as the one on the face, but it is noticeable.
I collected thousands of artifacts during the 1980's, in Arizona. I collected hardstone, metates, mortars, arrowheads, axeheads, pots, beads, shell jewelry, etc. And, I think that the object that you found *could be* an artifact. The problem is that the photos that you are showing do not do the object justice.
You are using unnatural lighting indoors, etc. One cannot see the texture of the groove, the interior sides of the groove, etc. If the bottom is full of divots, then I can see why a person may conclude a fossil fell out. On the other hand, if the groove shows signs of scoring, or grinding, then I would have to lean towards man made. The problem is that the photos do not sufficiently show the details. Also, I would agree that an experienced collector could easily give an informed opinion, if he saw the stone in real life. But, from these photos?
Please post better photos, with natural lighting that show the details inside the groove.