Wendy, there are a number of different simulant opals out there, and
a some claim to be synthetic opal, but there have been a couple law
suits over that claim. Anyway, some of these opals are plastic, and
as such are not very hard or long lived in jewelry. Some look very
convincing though. The glass simmulants such as Slocum stone should
be treated like glass with similar wear issues.
The Gilson and Inamori opals, the synthetics, are about the same
hardness and toughness as natural opal. The Gilson is very heat
tolerable though. A fellow opal dealer’s wife uses it with PMC and
has “never had one crack” which I find amazing, but I have no reason
to doubt her.
Hope this answers your questions.