A good appraisal will always identify the market the item was
placed and the approach to valuation. This isn’t my opinion,
it is from the Appraisal Institute’s Uniform Standards of
Professional Appraisal Practice (USPAP).
Real property (real estate) appraisers are testing on this as a
matter of law. Personal property appraisers (including jewelry)
may be tested as part of an ASA/ISA training course. Many
organizations also offer USPAP-only courses and tests many times
You can look for an appraiser who has taken USPAP (it is updated
every year, so you have to keep taking the new test) test. Those
people are likely to advertise the fact, and say in their
literature that they “adhere to the standards and practices as
described in USPAP.”
Furthermore, if one wants more about how one’s opal
was priced for the appraisal, (though it should all be documented
in the report), one needs only to ask during the appointment, or
later with a follow up phone call.
Despite the good work of the Antiques Roadshow, many people are
not adequately informed about what an appraisal is and includes.
Though gemologistsalone may be able to identify the opals, without appraisal training the valuation of the opal is very subjective and in a lot of cases the appraisal is done in a way that you can not trace the methodology used to value the item.
I would remind eveyone again that you need someone who is a
gemologist and has appraisal training, and USPAP, and has
experience with opals, or has an opal expert with who to consult.
(Why not go directly to the opal expert yourself? Because then
you would have a verbal opinion, not a written appraisal, and not
a gemological evaluation.)