Certificated diamonds are diamonds that have been scientifically graded under laboratory conditions. If the laboratory is a reputable one this means that there is no doubt when it comes to the grade of the diamond (see my diamond grading page for more info on the grading system: http://ringjewellery.co.uk/diamond-grading.htm).
So why go to all of the trouble of sourcing a certificated diamond? Well it's simple... if your friendly (or not so friendly) local neighbourhood jeweller gives his or her opinion on a diamond's quality they may be a little 'generous' in their conclusions. This is because they want you to buy the gem & telling you it's a great quality might just sway you. Now I'm not saying everybody that sells non-certificated diamonds is dishonest but this practice is way more common than you might expect. If you buy a laboratory certificated diamond then you know the true grade & value of the diamond.
The most highly respected diamond lab in the wolrd is the GIA (gemological institute of America). Here's their website: https://www.gia.edu/lab-reports-services/diamonds/index.html The way the GIA grade diamonds goes far beyond the 4 C's (carat, cut, colour & clarity). They look at a diamond in extreme detail as this sample GIA certificate shows:
A certificate like this shows how well a diamond has been cut & polished, how symmetrical it is & even shows a serial number that has been lazered on to the diamond to ensure that the paperwork matches the stone!
When I began working in the jewellery traded the only laboratory graded diamonds were very, very large ones! Now all that has changed & rightly so in my opinion. Although it's still not economical to pay a lab grade a very small diamond, as soon as you are looking for a stone over 0.30ct then you should ask your jeweller for a certificated diamond. It will give you (&your insurance company!) great piece of mind & will be very informative when it comes to the details of your specific stone.