It looks natural agate to me, too. Unfortunate that it didn't have a redder interior, but that's also characteristic of natural northwest carnelian agates - some have more red-orange inside, some less, than the outside reveals. There is some plume agate with carnelian colors in Chandler Mt. area agates, too (see photo below, which is old Moore Ranch).
All that said, I don't recall having seen pieces with a smooth brown side - almost looks like paint in your photos - though nearly anything's possible.
