The best approach in a mounted buying situation is to make a contingent offer on the diamond based on the seller’s representation or presumed grade. Have the diamond removed by a professional goldsmith who takes care not to damage the stone or the mounting; then weigh and re-grade it. Make sure that you inform the seller about who is responsible for remounting the diamond if a transaction does not occur. Often a ring’s crown (prong assembly) cannot be re-used, and therefore the cost of remounting may be more than anticipated. Make sure the seller is very aware that your tentative offer is based on a stated grade and weight and if incorrect, the price will be renegotiated.
Look in at least three different angular directions under each prong to identify possible hidden inclusions. In addition, while looking through the table tilt the diamond to expose the girdle reflection, watching for inclusions at the same time.