I am always asked this question mostly from people who are going to buy diamonds abroad. There are many different laboratories that issue diamond certificates around the world. There are the major ones like GIA, HRD, IGI and others like EGL and GAA. As a GIA gemmologist myself I am slightly biased as I know how strict GIA are in their training and their laboratories. All the larger diamonds are graded by seven grades and they take the lowest majority and not the highest majority as one would expect. GIA grade of scaling is really strict and in my opinion they have the strictest consistent grading system in the world and I can say that with confidence as I have seen thousands of grading certificates from around the world. The second best diamond certificate I have seen is HRD Antwerp. HRD did have inconsistences and some scandal associated with them a few years ago however, over the past number of years they have become reliable and consistent but they are probably not as strict as GIA.
I notice a lot of diamond rings sold “as much cheaper than conventional jewellers” use IGI certified diamonds. IGI has its own standard of grading which in my opinion does not come near to the GIA system of grading having examined hundreds of IGI certified diamonds over the years. Having said that one usually gets what they pay for. So, if one is getting a thirty percent cheaper diamond ring with an IGI certificate then that is what one has paid for. GIA certificates cost more to buy because they are rated higher.
The next biggest laboratory is probably EGL. EGL have agents or small laboratories all over the world. A number of years ago EGL were dropped by diamond world and the RAP lists (world diamond lists) as they were considered to have no consistency and rather over exaggerated certificates. In other words the certificates did not match the diamonds. They are still not listed on RAP although they are used a huge amount in South Africa. I personally do not rate EGL certificates for the above reasons. I have seen the odd EGL certificate which is consistent but, it is very rare. There seems to be too many different standards attached to all the EGL agencies.
There are numerous other certificates from various little laboratories all over the world some are ok and some are not worth the paper they are written on so, I would advise one to be very careful when choosing a diamond. For a number of years there were a lot of certificates being forged in Antwerp and also forged laser inscriptions so, it is very hard for the general public to be aware of forgeries. It is better always to pay with a credit card and have your diamond checked by a very experienced and qualified gemmologist who would be familiar with looking at forgeries. I have seen a number of diamond rings that have been valued by jewellers with no gemmological qualifications who did not spot what they were dealing with for example a forged diamond certificate or a forged laser inscription and when I told the client of this they have become very annoyed as the years have passed and it is too late to return to the original jeweller to challenge them. So, it is vitally important that you have your certificate and diamond checked by someone who actually knows what they are looking for and can spot it.