Occasionally we find agates and quartz nodules with this lovely, perfectly flat bottom. I imagine it happens when air gets trapped just below the surface of the cooling lava, but I'm speculating.
This nodule has that chalcedony skin in those different shades and patches.  It is dimpled all over but appears to be mainly quartz pocket. There is a partial flat bottom 
We usually find that the flat layers on the bottom of these have Waterlines in them. I don't see anything on this one but you could be lucky. It's a good nodule for your crystal display. I'm not sure if you would want to cut and polish it for cabochons. I'm not sure it is stable enough.
This is an Ayrshire agate found back in 2021, towards the start of the year. It was hiding in my cut it bucket.