and I recently did a whole series of articles on this subject. Since you're insistent check these for yourself:

Gemological Institute of America — “Diamond Buying Tips” — explains the role of independent grading reports and encourages buyers to ask for lab reports and to compare stones across channels (which allows brokers to compete on certified stones rather than brand markup).


GIA (paper) — “A Review of the Political and Economic Forces Shaping Today’s Diamond Industry” (GIA PDF) — an in-depth, nonprofit industry review showing how the diamond pipeline fragmented after De Beers’ dominance, creating multiple channels (manufacturers, brokers, secondary markets) that allow non-retail actors to offer comparable stones outside traditional retail markups.


Duke / Yale / academic law paper — “An Autopsy of Cooperation: Diamond Dealers and the Limits of Trust-Based Exchange” (Richman, law workshop paper) — academic analysis of how the dealer/broker system and private exchange networks function; useful for understanding why specialized brokers can efficiently source and trade certified stones at lower margins than full-service retail jewelers.


SSRN / academic case studies on pricing and online marketplace effects (e.g., “Profiting from the Decoy Effect: A Case Study…” & related SSRN work on diamond pricing) — academic work demonstrating pricing anomalies and how venue/market/channel affects price; supports the idea that different sales channels (brokers, online marketplaces) produce lower consumer prices for identical characteristics.

Federal Trade Commission materials / transcripts about pricing practices and transparency (FTC conference transcript on drip pricing & price competition) — while not diamond-specific, FTC work on pricing transparency shows how large retailers’ complex pricing and add-ons can raise consumer prices; by contrast brokers who sell stones “by the stone” and who focus on certified 4Cs reduce those hidden margins. This supports the structural explanation (overhead, add-ons, retail margin) for why brokers can be cheaper.


BCG / industry analysis — “The Future of the Natural Diamond Industry” (BCG PDF) — an independent industry analysis (nonprofit/analytical context) that documents value-chain margins and shows how value is eroded by added dealer/retailer steps; it quantifies the typical value reductions along the chain and explains how alternative channels (recycled, brokers, secondary market) can capture value for consumers. (Used here for value-chain evidence rather than retail marketing.)