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A certificate from a respected gemological laboratory is considered the final word on the quality of your diamond providing important information about it.
The purpose of a diamond certificate is to analyze and put down in writing the specific characteristics of the diamond like dimensions, carat weight, clarity and color, as well as other measurements and/or grades related to cut almost like making a fingerprint of it.
Which diamonds have certificates?
Not all diamonds have certificates. That's because:
Only loose diamonds, meaning those not yet set or removed from the setting are taken by the Gemological laboratory for certification. Combined with the fact that it is a time-consuming and expensive process, one can understand why only the larger diamonds (over 0.50 carat) that rank high in color, clarity, and cut are generally certified through labs.
Why are certificates important?
The certificate helps ascertain the diamond's market value by defining its characteristics according to a predefined set of rules and standards. Small variations in quality translate into big monetary differences. In consequence, the diamond certificate becomes an important tool on which to base your purchase decision.
Are all diamond certificates the same?
The loose diamonds offered by Wonder Jewelers come with certificates from one of these independent laboratories:
- American Gemological Society (AGS)
- Gemological Institute of America (GIA)
- European Gemological Laboratories (EGL)
Each of these is well-known and applies slightly varying standards/interpretations of the diamond grading traditions. While AGS and GIA are considered more conservative than the EGL, the differences are small. A diamond with a GIA or AGS certificate will cost a few percent more than a similar EGL certified diamond.
At Wonder Jewelers, it's mandatory to inform from where the diamond has been graded and the diamond is priced accordingly.
What's on a diamond certificate?
There are different parameters that find mention on a diamond certificate and leading laboratories evaluate the diamond on similar information:
- Cut: diamond shape and style, e.g., round brilliant cut
- Dimensions: measurements of the diamond in millimeters, e.g. 6.49 - 6.51 x 3.80 is the smallest - largest diameter and the total depth of a round brilliant diamond
- Carat: weight of the diamond to the nearest hundredth of a carat
- Proportions: Depth % (round diamonds) and Table % (round diamonds): in proportion to the diameter
- Girdle: range of girdle thickness, e.g. thin to medium
- Culet: appearance of the culet facet, e.g., very small
- Finish: Grades for polish and grades for symmetry ranging from poor to excellent
- Clarity: clarity of the diamond from under 10X magnification
- Clarity Plot: a diagram of the approximate size, location, and type of inclusions as through a microscope
- Color: color grade, usually the GIA 'D-Z' scale
- Fluorescence: color and strength of fluorescence when the diamond is exposed to UV light
- Comments: additional relevant information not mentioned elsewhere in the report
This sample certificate is from GIA:
Difference between a certificate and an appraisal
A Certificate lists the physical characteristics of a diamond that facilitate determining its market value; it does not purport to provide the monetary vale.
An Appraisal attempts to provide as accurate a market value for the diamond as possible, keeping in view the properties mentioned in the certification.
Remember when you pay less for a diamond appraised higher simply means what you pay is the price and the appraisal is meaningless (from being inflated).
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Diamonds 
