I don't think that's Agate, if it were there would be much better defined banding, and less opalescence allowing for the almos 3D look to it, as it appears there are inclusions of various impurities, within an otherwise clear, or possibly yellow transparent stone.
I can see why it would have caught one's eye!! What does the reverse side look like? Is it also polished, or is it flat like a Cabochon? I would also like to see side views, as well, or if you could supply more info, like is the clarity I am mesmerized by the more I look at it an illusion of the photo? Or does it in fact HAVE clear areas with the swirls of colors, and the "black smoke" and clouds? ALso are their any filaments running through it?
Sorry, but a single photo is not much for something with so much character! :-) Plus I'm no expert, but I've been around minerals and gems for a long time! I think I have been looking at this too long now because...I could SWEAR U see the letters HCL in the top left quadrant! As well as a dog's face and a cat (B&W) a little in front of the dog, maybe a JR Terrier...together they kind of merge in the stone looking like a Cow's face at first. LOL!
Okay time to lie down and medicate...err...meditate..for better vision, or maybe not such acute vision if those images are really reflections on the polished surface! LOL!
Good luck figuring it out!