u/50percentopacity

So I was out in my back yard today, watering a new tree that was just planted. We recently have had a bit of work going on back there including the tree and some raised beds. I happened to notice a rather large clump of overturned soil with a rock sticking out of it in one of the beds. So I picked it up, not thinking too much about it, but curious why this rather large rock was buried in my yard. I proceeded to wash it off when I discovered what appeared to be a smooth, polished side. I took it inside to the sink so I could scrub all the dirt and mud off of it. No idea how someone lost this clearly collected specimen. No idea how it got buried in my yard.

My location is northern Colorado but I doubt this rock is local.

What have I found?