There is a difference between a diamond certificate issued by the retailer and a certificate issued by an internationally recognised laboratory.
According to gemmologist Karl Micallef, of the Classic Group, who last September graduated with Distinction, obtaining a Diploma in Diamond Grading and Identification from the International Gemmological Institute (IGI), one of the reasons for the difference is that the laboratory is not selling the stone but only grading it.
Internationally recognised laboratories, like the IGI, the High Diamond Council (HDC) and the Gemological Institute of Amsterdam (GIA), have no commercial interest in the stone's value as they are paid according to the stone's size, not its value.
"Grading diamonds is a complicated task," Mr Micallef told The Sunday Times last week. "This is the only way that you can put a price tag to a particular stone, knowing that just one grade can make a huge difference of value, especially when we are grading stones that are larger than a third of a carat."
What is more important, he added, is the strict grading rules that are standard throughout the world and recognised by all the important jewellers around the world.
"Since the grading of a stone is based on gemmologists' opinions and judgment of the qualities contained in a stone, these serious laboratories apply three separate independent opinions from three different geologists to reach the final conclusion about a stone... not to mention the sophisticated, high-tech instruments used to determine the individual stone's authenticity and characteristics."
A stone's cut is the only element that man contributes to the diamond, he pointed out, and only the slightest variation of a facet can enhance or reduce the value and final effect in a polished stone.
"Therefore these instruments are of vital importance to measure a polished diamond for perfection. It is to be realised that this is a very specialised business that requires huge resources both in finance and knowledge, and therefore no independent can afford to have such a set-up.
Asked whether accurate grading was so important to the stone's value, Mr Micallef said: "It is not a must to have a certificate; ultimately, what is important is that your jeweller of confidence can assure you that the price you paid for your diamond is a fair price.
"On the other hand, when a certificate is produced, you eliminate all doubts that the new owner may have and it is the best guarantee that the price you are paying is equivalent to the stone's value and quality."
To ensure that the stone analysed is the one set in the ring, Mr Micallef said stones are either sealed in a plastic packet with the details on the packet and the same details on a Diamond Report, which then requires that the owner is present during the mounting of the stone; otherwise the certificate number can be inscribed on the stone's girdle.
"This method gives a perfect assurance that the certificate accompanying the stone is always authentic," he said.