Is this a ancient rock tool? Manmade or ?

I have pictures of this very intriguing rock, that seems to me to be very old, and looks to be manmade. It concave, and about 8" tall, and 5" wide. With a flat bottom, so it can be sat upright. I use it to pound meat flat.
I can send pictures, if anyone wants to see. The only thing close I can find is pictures from Catalina Island from a site dated 5,000 years old.
Tamara