Judging the Quality of Diamonds

To begin judging the quality of diamonds you have to have an undrestanding of the 4 C's. These are cut, clarity, color and carats. Each one of these has an effect on the cost of the diamond and fine diamonds will do will in all four areas. If three are excellent and the forth is poor then the value of the stone plummets. The diamond shape doesn't necessarily add to the value of the diamond, but does help to add some variety.

Most diamonds are considered to be ideal if they are transparent, or colorless. Diamonds that are rated as colorless are the most expensive of diamonds that are readily available. (Fancy naturally occuring blue diamonds, or other colors, are rather rare and difficult to find. Treated diamonds are more common.)

The colors of diamonds are rated with an alphabet grading system that starts with “D” and ends with “Z”. Completely colorless diamonds fall in the “D” and “E” grade, while colored, fancy diamonds are at the other end of the alphabet, at “Y” and “Z”. Here's the Blue Nile page on diamond color.

There are two main ways that jewelers’ use when judging diamond quality based on color. They may pull out a set of reference stones, and find the closest match between the diamond and the reference stone to determine its letter grade. Alternatively, the jeweler may use a colorimeter, which can electronically measure the color of the stone and determine its grade within 1/3 of a grade.

The clarity of a diamond refers to whether or not the diamond is clear all the way through, or whether it has tiny flaws and cloudy areas within the gemstone. It’s actually very rare to have a diamond with absolute perfect clarity and no flaws, but most diamond flaws are not visible to the naked eye. These flaws can only be seen with a magnifying device and only properly interpreted with suitable training.

The Gemological Institute of America (GIA) uses a magnification of about 10x when judging diamond quality based on the clarity. There are treatments available to make a diamond appear nicer than it actually is, and the treatments should lower the overall price of the diamond so it is important to do your research before purchasing a diamond to be sure you are not spending more money than you should. Here are some remarks by Blue Nile on diamond clarity

Carat is the term we used to describe the weight of a diamond gemstone and it's a lessor factor in the cost of a diamond.. One carat is equivalent to 200 milligrams of weight. so 10 carats = 1 gram. A 10,000 carat diamond wouild be 1 kilogram in weight. The biggest diamond ever found was, in rough condition, about 40 grams (400 carats.) Head off into outer space though, and you might find something bigger...

A single diamond with a high amount of carat weight is more expensive than the equivalent amount of carats in multiple stones. This is because as the size of the stone increases so does the rarity. Very large diamonds tend to fall into the priceless category.