HOW ARE THEY MADE?
The sphere is turned on a lathe then conical holes are 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 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 outer layer is twice as thick as the inner layers. This is because
the two outer layers are fused together. The reason for this is that
the outer layer with the dueling dragons is carved so deeply that if it
were free moving it would be too fragile and shatter, so it has to be
fused to the 2nd layer to make it strong enough to avoid breaking. 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 - an 18 layer ball will have 16 movable layers (including
the center ball) plus the 2 fused outer layers.