The sphere is turned on a lathe, conical holes are then drilled towards
the center. The maker has a set of "L" shaped tools, the one with the
longest upright has the shortest cutter, and the one with the shortest
upright has the longest cutter. Starting with the longest tool he
lowers it to the narrow bottom of each hole in turn and rotates it to
cut the innermost ball free. Then he gets the second tool, which does
not reach quite so far down the hole, but can cut a wider arc and separates
the second ball. He continues working from the innermost to outermost
shell. For obvious reasons, usually only the outermost balls are
elaborately carved.

The two outer layers are fused together making it twice as thick as the inner layers. The reason for this is, the outer layer with the dueling dragons is carved so deeply that if it were free moving it would be too fragile and shatter. Therefore it has to be fused to the 2nd layer to make it strong enough to avoid breakage. The very inner ball at the center is also counted as a layer making up the total number of layers. Therefore counting the center ball and the other moveable layers will make up the total layers of the ball minus two layers, those remaining two layers are the ones fused together making up the outer shell.

For example - a 10 layer ball will have 8 moveable layers (including
the center ball) plus the 2 fused outer layers.