I studied works of Chinese stone carvers and came across balls that the Europeans used to call 'Devil's'. Just because they didn't know how they were made.
These are carved balls, but! with 5 to 20 other balls inside and they move freely. And each piece was covered with exquisite carvings that never repeated.
The most complex thing is balls with amazing carved lace inside — one awkward movement and the pattern is wasted, the blank is thrown away.
The peculiarity is in round holes — perfectly smooth that meet each other exactly in the centre.
Then a markup of future balls is made with a ruler. Balls are cut out from the centre, with curved cutters, relying on experience only — a master does not see the surface. Stunning accuracy!