I will only address the lapidary/gemology part of your questions. A
well cut stone will not have any "window" to the rear. All light
enters through and reflected back through the table (top facet) of
the stone. Thus you should not see any of the skin from the wearer
through the stone. Open back does not increase the
reflected/refracted light coming from the stone.
If your stone has a window, then it should cost less as it was not