My brother does stuff like that.
Here's a blurry image of one that she gave me (after I wore down the polish a bit). http://i.imgur.com/IhvgT.jpg
I never realized how unique the stones were until I was in my 20s and discovered most people know nothing about them. The buckets are long gone and one of my biggest regrets is not having claimed any before my grandparents moved back down to southeast MI.
Of course the rock is millions of years old, but the fact that it has the shape of a shell embedded in it from millions of years ago just makes the fact so much more real.
Wikipedia pages about predictable things are not interesting, but when the topic is uncorrelated with anything else, they're fine.
("TIL A Michigan rock collector found an impressive 93 lb. Petoskey stone in Lake Michigan in 2015, only to have it promptly confiscated by authorities. The stone violated a Michigan law that states no more than 25 pounds of rocks or minerals can be taken from the Great Lakes per year.")
I'm guessing that someone reading that reddit post found the phenomenon interesting and submitted its Wikipedia page to news.ycombinator.