I think they may be natural. Looks to be quartz or fossil inclusions. You can see on the edges of the second one where the fossil or whatever it is is softer material and worn away. Harry Pristis may even recognise the material he is a fossil expert. Pretty cool looking though.
The groves are the same all the way around both sides. I always thought they might be something from a necklace or something of the sort. They were found in western colorado just about a mile above fort uncomphagre.