Buying a diamond is one of the few significant purchases most people make with no product knowledge, in an emotional state, from someone whose expertise vastly exceeds their own. That is precisely the situation an investigator is trained to be suspicious of.
Rule One: Cut Above Everything
Of the four Cs, cut is the one that determines whether a stone actually looks alive, and it is the one buyers underweight most. A well-cut stone a grade lower in colour will outperform a poorly cut stone with better paper nearly every time. Spend on cut. Economise on the grades nobody can see across a dinner table.
Rule Two: Colour and Clarity Have a Point of Diminishing Returns
Above the point where inclusions are invisible to the unaided eye, additional clarity buys certificate prestige, not appearance. Similarly, colour differences of one grade are effectively invisible in a mounted stone to anyone who is not holding a master set. Know where you stop paying for something you cannot see.
Rule Three: Verify the Certificate, Not Just Read It
Reputable laboratories publish report verification online. Check the report number against the laboratory's own database and confirm that the stated weight, measurements and any laser inscription match the stone in front of you. Certificates are swapped, altered and issued by laboratories with conveniently generous grading. The document is a claim until you verify it.
Rule Four: Ask Directly About Treatment and Origin
Fracture filling, laser drilling, HPHT colour treatment and irradiation are all common and all materially affect value. So does lab-grown origin, which is now excellent, inexpensive and entirely legitimate — provided it is disclosed. Ask the question in writing and keep the answer. Non-disclosure is where the fraud lives, not in the treatment itself.
Rule Five: Compare Before You Commit
Prices for comparable stones vary widely between retail, trade and private sale. Anyone pressuring you to decide today is managing your emotions rather than serving your interests. There is no diamond that will not be there next week, and if there genuinely is, there is another one exactly like it.
Rule Six: Get an Independent Appraisal
An appraisal from someone with no interest in the sale is the cheapest insurance in the transaction — and you need one anyway to schedule the piece on a policy. Estate and inherited pieces especially: family attribution is frequently wrong in both directions, and pieces dismissed as costume have turned out to be significant more than once.
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Dan Ribacoff is a licensed private investigator and the founder of International Investigative Group, a certified polygraph examiner, certified firearms instructor, GIA Graduate Gemologist and the author of I, Spy: How to Be Your Own Private Investigator. He works from offices in New York, Boca Raton and London, and has appeared as a polygraph expert on American and British television.