If you are working with an asymmetrical piece for a bezel-set stone,
you may just use a fine scribe or a sharp needle to locate the
suspension point. Place the up-turned sharp point at the inside base
of the bezel to suspend the mounting so that it swings freely. Adjust
the position as needed and mark.
To locate a centerline, balance the piece and place a mark at 12:00
and one at 6:00. The line connecting the points is through the center
of gravity of the piece. This facilitates proper placement of pin
findings and clip style earring findings.
If the stone is also irregularly shaped that will affect the balance.
Carefully tape the stone into proper position on the reverse of the
mounting. Protect the surface of the stone if necessary as the top
surface of the stone will be against the back of the mounting.