Uniform striations...can't find an id

[Snee]

It's nothing special, but I was curious. I tried researching it and have come up with nothing, so now I'm both curious and confused.<br>I'm in central Missouri and found it at a creek. I'm figuring it's limestone. I'm just really curious about the striations. They are uniform in direction and go all the way around and through the rock. I'm figuring it's geologic. Does anyone have any information? Thanks!

 

[Snee]

Sorry for the double post. I'm on my tablet at work. It did something weird and I can't figure out how to delete the one without pictures.

 

[stdenis_jd]

looks like it could be a slab of a fossilized coral, mostly silicified. Could be wrong