Recommended Certificates:
1. GIS Certificate (Gemological Institute of America)
2. AGS Certificate(American Gems Society)
3. EGL 

After understanding principles 4Cs of diamond, now we go to practical step for choosing diamond in value. Frankly speaking, diamond looks all the same after effect of light, photoshop, and other special effect. You cannot tell the difference when judging by naked eye, not to say though PC screen. You do not need to master and memorize everything for the good certificate, but it is good if you can choose the best value of diamond you want from various percentage and number in certificate by referring to qualified range as below.

4 Cs

This is number to show whether the cut is ideal, too deep or shallow. A good cut needs a balance and good proportion of depth, measurement, table, cutlet, girdle. A diamond won’t present its brilliance without good pavilion angle and crown angle too. 

1. Depth %
There is direct linear relationship.
Acceptable range: Every diamond shape has its ideal depth % to reflect most its brilliance.  For example, a good depth % for round diamond should be in range of 58.0% to 63.0%. See also diamond proportion for different shapes.

2. Table %
Size of table/width of diamond. Generally, the smaller table in % is, the more fire of light it plays. A larger table % has less fire with larger diameter size when looking from top.
There are two types of brilliance of light: life and fire!
White light is reflected straightly from table, which we called “life of diamond”. Fire is come from crown facet that surrounds the table, and it is multicolored.  

Acceptable range: Grading standard of table % varies in shapes.
Let’s take an example: A good table % for round diamond is between 53% to 62.5%. See also table % for other shapes.

3.  Girdle
Range: Extremely Thin, Very Thin, Thin, Medium, Slightly Thick, Thick, Very thick, Extremely Thick. The thickness of the girdle may not be even due to design and range.
Acceptable Girdle: Thin; Thin to Medium; Thin to Slightly Thick; Thin to Thick; Medium; Medium to Slightly Thick; Slightly thick; Medium to Thick. Avoid extreme thin, very thick and extremely thick girdle.
Reason: For extreme thin girdle, it is prone to chip along the girdle.  Since thick girdle not adding any value to diamond, if you choose a extremely thick girdle means you pay for the unnecessary weight.

 4. Cutlet
Range: none, very small, small, or medium, large or very large.
Acceptable cutlet: none, very small, small, or medium.
Reason: When looking from top of the diamond, the dark spot may be visible for large or very large cutlet. And it slightly reduce the perfectness.

 5. Polishing
Range: Poor, Fair, Good, Very Good, Excellent
Acceptable polish: Good, Very Good, Excellent
Reason: Polish measures quality of finish on facets. Like streaks remain after car waxing Polishing lines might remain on the surface caused by polishing wheel after diamond cutting process. It can be seen by 10X magnificent microscope or naked eye. It has not much impact to diamond’s appearance but the price is different by grades.

 6. Symmetry
Range: Poor, Fair, Good, Very Good, Excellent
Acceptable Symmentry: Fancy looking diamond, grade of GOOD is preferred, while for round brilliant diamond, very good and above is preferred.
Reason: After looking at the shape, you may observe the symmetry of facets. It is the balance and alignment of facets and shape. The possibility is that the girdle may not perfectly round, facets maybe distorted, sloping table or points not symmetrical. It has not much impact to diamond’s appearance but a terrible misalignment can diminish the reflection and fireworks. The price is different by grades, which is excellent, very good, good, fair and poor. 

 7. Fluorescence
Range: None, Faint Blue or Medium Blue, Strong, Very strong yellow or blue
Acceptable range: None, Faint Blue or Medium Blue. (depends on the color grade
Reason: Fluorescence often adds value to yellowish color stones as it gives the stones a whiter, brighter appearance. While strong fluorescent could cause pure colorless diamond to appear milky and oily in regular light. Know more from budget on fluroresence.

8. Comments
Normally comments are not significantly impact on the value of diamond. GIA does not record crown and pavilion degree on the specification, although they carry important message about 3D of diamond. Notice if they comment on crown angle and pavilion angle. Check if the angle is within the range of standard.

CROWN ANGLE — is the angle formed where the girdle intersects with the crown (i.e., the top part of the diamond). It is the point of entry and exit of white light to the diamond. If the crown angle is too shallow or too deep, the fire of the diamond is diminished.

Ideal Crown Angle: 33.7 – 35.8
< 32: Spread-out. Table too large and girdle is thin. This diamond structure prone to chipping.
<35: Diamond gains unnecessary weight on top, and eat up some weight from somewhere. Hence it reduces the overall size for same carat.

This is not the standard of crown angle as ultimately we need to comply with pavilion angle, which play more important role in brilliance.

PAVILION ANGLE – This angle is the intersect of crown with cutlet (depth). It is the most important part of diamond to reflect light. White light passes through the table and facets of crown, and then reach pavilion. The best angle enables the facets of pavilion to reflect light to the opposite pavilion and lights reflected back up to table.

Ideal Pavilion Angle: 40 – 41.5
< 40 or > 41.5: If the pavilion is too shallow or too deep, lights leak out and pavilion will not reflect light perfectly it leaks out. There will be loss of brightness for this range.