Any idea what this is?

[Mlee]

I found this stone at a flea market a couple of weeks ago, no idea what it was but for $2 I thought what the heck. Last night I got it out and looked at it. It was black and very concave so I thought I would clean it up a bit. Here's what I ended up with, not completely flat but almost. In the second picture, you can see that it's obviously natural.

I'd also like to know an approximation on the grit if possible.

Thanks for any help. I'm new at this, hopping to learn something.

 

[joamo]

Looks like a Washita to me.
Here is some discussion of Washitas

 

[stone and strop]

My guess as well.

 

[Mlee]

Looks like a Washita to me.
Here is some discussion of Washitas

My guess as well.

Thanks for the link, that's what I wanted to know.

 

[duke762]

Nice score! Love the backside!

 

[SliceOfLife]

Washita. Very hard one. Back is likely due to it being from a wood box originally which it was ripped out of.