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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 moveable layers (including 
the center ball) plus the 2 fused outer layers.