The information below refers to GIA and AGS graded diamonds. There are many labs in the industry, some with different ratings but, the two I recommend are GIA and AGS.
Diamond Carat
Carat is simple and easy to explain. It’s simply the finished weight of the diamond, once it’s been cut. It will be measured and weighted to give you the total.
Diamond Color
You’ve probably heard of a G SI1 or maybe a D VVS, the individual letter represents the color grade that has been given to the diamond. In the 1950’s a man by the name of Richard Liddicoat developed a grading system to give a grade to each diamond and it’s color. The scale ranges from D-Z (D being the whitest diamond and Z will be the warmest of diamonds) and is still used today. Here is a chart from GIA (make the GIA link to this please https://www.gia.edu/gia-about-4cs-color) to give you an idea of the actual color scale.
Color is present in the diamond rough and is caused by the natural, environmental influences which were present at the time it formed in the Earth. When a diamond is given the grade for color, the diamond is actually graded from the pavilion (pointed end) side down, not the face up view which is how we view diamonds. The diamond is graded from this side to provide a true color evaluation to the diamond. Many other factors will determine how the diamond actually faces up.
Another important factor which will influence how your eye perceives the color of the diamond is the cut. You can take two G colored diamonds. If one of the diamonds is cut incorrectly and one had ideal proportions and angles, all working together correctly, the Ideal cut will face up more white and bright than the other of the same grade.
Depending on your personal preference and what you want most, color will be one of your deciding factors.
Diamond Clarity
Clarity refers to the amount of natural characteristics that are present in the diamond. You have many different types of inclusions that can occur in and on the diamonds, while they are forming in the Earth. The clarity rating scale goes from FL (Flawless) to I3 (Included). If you want a more in depth review, here is another link to the AGS (make the AGS link to this: http://www.americangemsociety.org/page/diamondclarity) clarity ratings.
Types of inclusions include: Crystals, Clouds, Feathers, Twinning Wisp. Here is a link defining the different types: http://4cs.gia.edu/en-us/blog/diamond-inclusions-defined/
Diamond Cut
This will be your most important C! The cut of the diamond will impact all areas of the diamond. The more precise the cut, the more sparkle and brilliance you will get out of the diamond. All angles and proportions need to be working well together in order to return the light correctly. When you have an ideal cut diamond, the will maximize size, make color appear brighter and can drown out the appearance of inclusions, improving all the C’s.
In order to get the cut classification, GIA collects the details based on the polish, symmetry and proportions. AGS, takes the cut grading a bit further with a few light performance test, insuring the light is returning correctly, maximizing the scintillation and sparkle.
Here is a link to the AGS website for more details: http://www.americangemsociety.org/page/diamondcut
Here is a link to the GIA website for more details: https://www.gia.edu/gia-about/4cs-cut
Let me compare GIA EX and AGS Ideal, short and sweet. AGS ideal holds a very strict requirements in order to reach the highest level of grading. With GIA, you would need to check a little more in depth when selecting you triple EX diamond.
Here is a more in-depth link explaining the differences between GIA EX and AGS Ideal:
https://www.briangavindiamonds.com/news/ags-ideal-vs-gia-excellent-cut-diamonds