Looks like one to me too. I have found a few fist size ones over the years. My dad has been the lucky one. He has one that weighs 10 lbs and is the size of a cantaloupe.
I like to tumble polish mine, or the easy way is to display them in a clear jar full of water.
.. I polish all my agates smaller than golf ball. My tumbler is too small for the bigger ones. I soo badly want to cut it open but I prob will just leave it whole.