Buying Estate Jewelry at Auction: What to Check Before You Bid

1. Gem reports: what they tell you, and what they don’t

A grading report from an independent laboratory describes a stone’s characteristics. For diamonds, the best known is the Gemological Institute of America (GIA). You will also see older reports from AGS Laboratories, the American Gem Society’s lab, which issued diamond reports until it closed at the end of 2022. Other labs exist, and grading standards are not identical from lab to lab.

• Ask for the report number and look it up yourself. GIA offers an online Report Check that shows the report details.
• Many lab-graded diamonds carry the report number laser-inscribed on the girdle. Ask whether the inscription was checked against the report.
• Compare the measurements and weight on the report with what the house states. Mounted stones can only be estimated, so differences need an explanation.
• Note the report date. It describes the stone when it was graded; chips or damage since then will not appear.
• A report is not an appraisal and does not state a value.

If a significant stone has no report, the estimate should reflect that. Ask whether the house will allow time to have it tested, or whether the price could support sending it to a lab after purchase.

2. Treatments, lab-grown stones, and simulants

Many gemstones are treated to improve their appearance, and treatment affects value. Common examples include heating of sapphires and rubies, oil or resin in emeralds, and fracture filling in diamonds. Lab-grown diamonds and gemstones are chemically real but are valued very differently from natural stones, and simulants such as cubic zirconia or moissanite are different materials altogether.

• Does the description say “natural,” “lab-grown,” or say nothing? Ask if it is silent.
• Has any treatment been disclosed, and is it supported by a lab report?
• For important colored stones, specialized labs can report on treatment and sometimes geographic origin. Untreated stones with documentation generally command more.
• How did the house test the stones? A quick tester is not the same as a laboratory report.

3. Hallmarks and metal

Stamps on the shank, clasp, or back of a piece often indicate the metal and sometimes the maker. Common marks include:

• Gold: 10K or 417, 14K or 585, 18K or 750 (the numbers are parts per thousand of gold).
• Platinum: PLAT, PT950, or 950 PLAT.
• Silver: 925 or STERLING.
• Gold-filled or plated: GF, GP, or HGE. These are not solid gold.
• British and many European pieces may carry formal hallmarks showing the assay office and a date letter.

Stamps can be worn away, missing after resizing, or simply false. “Tests as 14K” means the house tested the metal; it does not mean the piece is stamped. For signed pieces, look for the maker’s signature and serial number, and ask whether original boxes or receipts are included. Phrases like “in the style of” mean the piece is not being attributed to that maker.

4. Condition: look closely, then ask

Photos flatter jewelry. Ask for a condition report and, if you can, look at the piece with a loupe at preview. Things to check:

• Prongs: worn, thin, or bent prongs can let a stone fall out.
• Stones: chips, abrasions, cloudy or replacement stones, and any missing stones.
• Metal: thin ring shanks, cracks, solder marks from resizing or repair.
• Clasps, hinges, and safety catches: do they close securely?
• Pearls and beads: when were they last restrung?
• Enamel: losses or repairs.
• Size: can the ring be resized safely, and what would it cost?

5. Appraisal value is not an auction estimate

This is one of the most common sources of confusion. An insurance appraisal usually states a retail replacement value, meaning what it might cost to buy a similar new piece in a store. An auction estimate is the house’s opinion of a likely hammer-price range in the secondary market. The two numbers answer different questions, and the appraisal figure is often much higher.

• Do not treat an appraisal as proof of resale value, or a low estimate as proof of a bargain.
• If you want an independent valuation, use a qualified gemologist-appraiser with recognized credentials who has no stake in the sale, and be clear whether you want replacement value or fair market value.
• Compare like with like: recent auction results for similar pieces are a better guide to auction prices than retail tags.

6. Budget for the whole cost

Add the buyer’s premium, sales tax where applicable, insured shipping, and any sizing, cleaning, or repair the piece needs. Then set a hard max bid. After you win, add the piece to your insurance promptly; see after the hammer falls.

Buying a watch as well? See the collector’s checklist for buying luxury watches at auction. For paintings and prints, see buying art at auction.