Estimate and sale are different
A pre-sale estimate is an expectation, not a completed transaction. A hammer price is the accepted bid; a reported sale total may also include the buyer’s premium. Check the auction house’s explanation before comparing numbers from different events.
Keep the original currency
Record the auction house, location, date, original currency and reported total. A US-dollar equivalent is useful context, but it reflects the conversion used at that time. Historical results should not be presented as current asking prices.
Compare like with like
Color, origin, treatments, clarity, dimensions, provenance and condition can change the comparison. A single exceptional sale cannot establish what a less distinctive piece is worth. Nor does an unsold lot necessarily say everything about the quality of the jewel.
Build your own reference file
Save the lot description, accompanying reports, condition information and the published result. Before bidding, understand the buyer’s premium, payment requirements and shipping arrangements. Questions answered before the auction are far more useful than surprises afterward.