BTW, I ended up using a local (Kent, WA) industrial 3D scanning/metrology company to get the scan. Literally named “Digital Scan 3D”, which is an incredibly on point name for the business.
They had an equivalent of the Solutionix D700 scanner that is made for scanning small objects. Beautiful output in a .stl format with very good resolution of even the smallest depressions and bumps. I can use it as a “slicer” to cut a 3d model of the setting, cast it, and it will perfectly fit the contours of the edge of this very irregularly shaped stone.
One-off cost was $285, which would be very pricey to do too often, but perfect for this particular situation. It was done essentially same day. Dropped of the stone, later they emailed me the STL file, went and picked up the stone the next day. With better coordination you could probably get it done while you wait, I just happened to drop it off at around closing time.