It is best to have the diamond appraised when it is not set. The appraisal will be much more accurate. Is this a diamond you already own or one you are considering purchasing?
A qualified appraiser/gemologist should have no problem appraising your mounted diamond.
There are limitations but estimated weight, color and clarity should be the minimum information on your appraisal.
Table %, girdle thickness, culet, fluorescence, polish, symmetry, etc. should be noted as well as any other visible characteristics.
The more detail the better. Also if you have a laboratory report, make sure that you mention that to the appraiser so information may be verified versus the appraiser''s findings.