vera - that ‘backing material’ is called ‘stabilizer’. it is necessary
for some varieties of turquoise because they are more friable & need
something to hold them together during the cutting, polishing &
setting stages (plus they add weight for selling by the carat/gram).
the stabilizers range from tar-like material to polymers. sometimes
they are too thick & you need to grind the backing down to a usable
depth. sometimes they are added simply as a precaution (plus they add
weight for selling by the carat/gram. ;-]) if you have been cutting &
polishing your pieces without disaster your material doesn’t need the
backing. if you have some doubts about your material you might
consider putting a backing bed of E6000 into the bezel before setting
the turquoise - it’s clear & doesn’t look as tacky as the
black/brown/gray stabilizers should it show. good luck -
ive