By placing glue on the back of quartz stones, it is effecting the
refractive index of the material. You are actually seeing the glue
thru the stones. There isnt a remedy to this ,except gluing in more
foilbacks. The foilback coating keeps the glue from showing thru.
Light bounces off of the ‘mirror’ side of the foil coating and back
to your eye. Rio carries a decent selection of foilbacks, but the
minimum orders can be tough sometimes.Ed