Authenticity is virtually impossible to determine from photographs.
You stating you found it is good proof of authenticity.
Some sites have been known to be salted in an attempt to fool or have items recently made brought into the collector world as authentic.
This item is not the classic shape of a Bi-concave gorget. To be submerged in water for how many years it should have been, one would expect to find evidence of submersion such as discoloration or build up of minerals. Slate is porous and the water over the years would have impregnated the material in different amounts depending on the minerals in the water and the density of the material.
During manufacture, after pecking to the desired shape, the holes are drilled and then polishing takes place. Often some peck marks are still visible even on relics with a very good polish. Look for peck marks on the relic with a loupe.
The holes on those I have found, pendants and gorgets, mostly then have a sharper edge around the holes with often wear and more smoothing in the are where the leather string would have worn against the material over time.
Some of the slate relics I have seen from Ohio have different holes configuration than many found in Indiana.
Using a loupe, look for mineral deposits and try to post close up photos of any found please.