2c...

One possible approach: Take appraised value, discount according to authorship (i.e. appraisal only as good as the appraiser), divide by 3 and you've got somewhere Why? Because many of those papers are written with the assumption that you will do just that (a polite formulation: it is a 'moral' gesture to overvalue 'cause this serves the appraiser's client's interests in the long run... which sounds sort of right for the appraisal issue stance, but uncomfortable for the larger readership of these documents, so to speak...).

One common caveat trespassed: appraisal and identification are not just two different things, but two things that must stay different ... (don't take my word for it, just consider that none of the major gemological laboratories places any valuation on their reports). Preposterous as this confusion might sound, it is often questioned and ...

There's quite a bit about appraisal practice and value on the gemology forum, most posts by professional jewelry appraisers. Illuminating read, methinks.