I have a beautiful Lapis stone pendant that is set into a really cheap and eroded setting. How can I save the stone? I have no way of finding out what the stone was adhered with.
First you need to identify how the stone is set--bezel setting, prong setting and/or glue.
If the stone has been mounted in a bezel style setting, you will need to loosen the bezel. This can be very difficult if not impossible to accomplish without damaging the stone. If the stone has been mounted with a prong setting, the prongs will need to be pried back to allow the stone's removal. To avoid scratching or damaging the stone, use a prong lifter. You could also coat the tool with Tool Magic® so any edges which might hit the stone as you work are covered, protecting the stone's surface. For stones set with an adhesive, you will need to break down the adhesive without damaging the stone.
Note: the following process was tested on natural lapis, but may damage some stones especially if dyed or otherwise enhanced. It is recommended that you test a stone for any adverse reactions to the acetone before attempting the following method.
Acetone (nail polish remover) will be affective in breaking down most adhesives. Soak the setting and stone in acetone for several hours and then check to see if the adhesive is breaking down. Lapis is not a delicate stone, with a Moh's hardness of 5 to 6, soaking in acetone should not affect the stone if it is real lapis. You may need to repeat the soaking process a few times before the acetone breaks down the adhesive enough to pop the stone out.