I don't know what kind of rock the cave is made from that you are in but around this area it's sandstone and in that sandstone we get iron rich layers that are harder than the surrounding material. Over time they erode out in plate like pieces much like the one in this picture.. they even have that wavy look on one side frequently. Also, the piece itself wouldn't serve any purpose that I can think of in terms of an artifact. It has an unusual shape but that's about it from what I can see. The key part of that being "from what I can see". There could be some lines etched into it like you were saying that could be from human hands... maybe you could scan that side? Maybe, if there are some lines it could have been used to abrade or prepare striking platforms for flintknapping, then again, a rough textured stone would be better for this vs. a smooth one like this. I am sure it's not any kind of preform or anything like that otherwise signs of human intervention would be more evident. Actually, I take that back, I suppose it could be a preform for a pendant or gorget or something along those lines but I think it's a long shot. Like I said, if you could get a good clear scan of the surface I think we could put it to rest one way or the other. Out of curiosity was there a lot of flaking debitage in the cave?
