I gave it a try! I tried to get the odd-ball perfectly circular cut and square dimensions, but I'm mostly just eyeballing it. Haven't tried printing it yet, but I have some nice red filament that I think is going to look good!

Here's a screenshot from inside FreeCAD:

https://f.toi.sh/rubygem-screenshot.png

A nice manifold solid:

https://f.toi.sh/rubygem.3mf / https://f.toi.sh/rubygem.stl

A terrifying non-manifold FreeCAD mess: (requires surface WB)

https://f.toi.sh/rubygem.FCStd

I love this idea!! Any luck finding an STL or at least a 3d model I can convert and copy your idea?

A quick search returned this: https://sketchfab.com/3d-models/gemstone-pack-68c4ec3dd23247...

Not _exactly_ the same cut, but might be good enough for you?