I may be way off beam here but it seems to me that there is some
confusion as to just what it is that is being set.
Fossil ammonites are as hard or soft as the mineral that they are
composed of. Varies from iron sulphide (not very stable) through to
silica (lasts indefinitely and is hard).
But, there are one or two places where really large fossil ammonites
are found in which the original shell has been replaced by minerals
which display a play of colour similar to black opal ie not really
black, but showing bright irridescence. This is sold in pieces up to
a few inches square, always rounded (domed).
I saw plenty a year ago in New Zealand made up into earrings and
pendants. As far as I remember there was gererally no glass covering,
though it may have had a coat of lacquer.