This looked like a true agate when we picked it up down in South Ayrshire. Agates from Scotland tend to follow similar recognised patterns tho, of which this rock has none.

There may be the ghost of banding just inside the skin but this is obscured by a bit of calcite scuff. Perhaps further tumbling by a lapidary artist would perfect the shine. We took it to 600 grit in our tumbler after finding the stone on one of our many rock hunting trips.