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Not recommended to buy off of certificate grading alone. Sometimes certificates don't paint a full picture and you should always ask the jeweler how does the stone look to them.
Diamonds don't have to be certified to be a nice quality stone and are often priced lower without a certificate.
GIA certified diamonds are the most sot after in the jewelry industry.
GIA (Gemological Institute of America)GIA certificate is highly recommended if you require a diamond with a certification. GIA grading standards are the strictest compared to other gemological institutes. Most jewelers base their diamond grading and buying off of GIA standards.
AGS (American Gem Society)AGS and GIA were founded by the same group of jewelry professionals. GIA was made for education and research while AGS was designed more for jewelry professionals. AGS and GIA have almost identical grading standards except when it comes to cut grading. AGS has set themselves apart from other gemological institutes because they have focused on the importance of the Cut of a diamond. AGS has developed the only scientifically vetted system for analyzing the quality of cut and light distribution in diamonds.
3. (IGI) International Gemological InstituteIGI is rated as a high quality gemological institute but is still not a huge player in the jewelry industry compared to GIA or AGS. Most diamonds graded by IGI are graded almost identical to GIA yet a lot of jewelers lack in the knowledge of IGI grading abilities.
EGL (European Gemological Laboratory)EGL is very well known in the jewelry industry. They are known for not being the most accurate in grading. Typically when jewelers buy a diamond with a EGL certificate they know the quality will be a couple grades lower than what they would expect compared to a GIA certified diamond. Prices normally reflect the difference in grading, you can expect to get a lower price for a diamond that has the same graded characteristics compared to GIA. EGL also has three different sectors. They have EGL USA, EGL Hong Kong, and EGL Israel. Each branch grades slightly different and you can expect that USA standards are slightly higher than Hong Kongs, and Hong Kongs grading standards are slightly hire than Israels.
Certificates are created for diamonds and other gem stones so a buyer knows what quality stone they are looking at.
Not all diamonds have certificates.Not having a certificate does not mean it is not a quality diamond however it may be a more biased grading compared to leading gemological laboratories who provide certifications.
Below are the leading gemological laboratories.There are other grading laboratories however the labs listed are most used in the jewelry industry. If you come across another certificate look up their reviews to get a better feel of their organization!