it can be done.
What you need is a plexiglass glue. it has the same light (hope I spell this right) refraction index (or close to) the jewel plastic.
This glue (2 component glue) if it hardens it looks and feels like plexiglass. (you could even cast a new jewl with it)
The reason this works, is that the glue also has a low density that makes it flow into the small grooves created by the sanding . The result will be a clear , maybe a bit blurry new jewel.
It might be needed to put the jewel on a spinning disk to apply the first layer. This to make the glue disperse evenly into all the grooves.
After that just brush it on thick and let it settle.
(be carefull with dust, it will show clearly if you use a backlite for the jewel later!!)