So he began doing research and found that odd-size stones — say 1.47 carats, not the complete 1.5 carats — cost a bit less and fit into the ring setting and his budget of about $15,000. But he acknowledged that he was out of his depth when it came to knowing why similar stones were priced so differently.

Despite different budgets, Mr. Taira and Mr. Fennerty shared a trait: fear of getting ripped off in the notoriously opaque diamond market. Both turned to Rare Carat, a two-year-old website that uses an algorithm to value different diamonds based on their qualities — size, cut or clarity, for example.

The site has also recently added a tool that finds the best price for a particular diamond, because diamonds from one supplier can be offered across multiple retailers at vastly different prices.

“Consumers are walking into stores blank,” said Apeksha Kothari, chief operating officer of Rare Carat, which aims to be the diamond version of Kelley Blue Book. “We want to empower our customer with information. They know so little about what they’re buying.”

Like the travel site Kayak, Rare Carat is paid on a per-click basis; it does not make money off the final sale of a diamond.

Two separate reports confirmed that buyers should be anxious when it comes to the price they are paying for a diamond. That exact diamond can often be found somewhere else for less.

Katja Seim, professor of business economics and public policy at the Wharton School at the University of Pennsylvania, used the Gemological Institute of America certificate number on diamonds to search for price differences on the three leading online retailers — Blue Nile, James Allen and Brilliant Earth. Her research found that on average the price for the same diamond can be 12 to 21 percent higher on those platforms than at smaller retailers.