The quality and value of a diamond is judged on four fundamental criteria known as the four C’s. The carat-weight, colour, clarity and cut. Some differences in quality are easily noticeable, others need specialist equipment and qualified graders with years of experience to assess and detect.

Accurate assessment of a diamond is only possible prior to being set. Once the stone is set even a trained grader cannot precisely determine colour, size and purity. Therefore quality and value can only be estimated. Minor differences in colour, clarity and weight can have a major effect on value. For this reason, laboratories only grade loose diamonds.

In addition to the four C’s, laboratories test for treatments, which enhance the colour and clarity of the diamond. Treatments such as fracture filling, laser drilling and high temperature hig pressure, significantly reduce the value of the diamond and should be disclosed.

The weight of a diamond is expressed in carats. 1 carat equals 0.2 grams. A carat is further subdivided into 100 points.

0.50 ct = 1/2 carat = 50 points.

Weight is the most obvious factor in determining the value of a diamond. But two diamonds of equal weight can have very unequal values, depending on their quality. Furthermore weight only indicates size. Depending on proportions two diamonds weighing the same may appear very different in size. Internationally, the weight of a diamond is mentioned to two decimal places.

Click here to view a diagram that compares the carat weight from the DCLA website.

Most gem quality diamonds fall within a range from “D” colourless to “Z” light yellow. Almost all have a trace of yellow, brown or gray body colour. The colour of a diamond is determined on the basis of a visual assessment by comparing it to a set of master diamonds under laboratory conditions.

The differences between colours are very subtle and graded according to the International Colour Grading Scale. With the exception of some natural fancy colours, such as blue, pink, purple, yellow or red, the colorless grade is the most valuable.

Due to inaccuracy and inconsistency, colour machines are not used to grade diamonds by any of the internationally recognised laboratories. The prescribed method for grading colour is by comparing the diamond to a registered master set.

Click here to view colour diagram chart from DCLA website.

Diamonds may have various inclusions or imperfections. The number and size of these determines the clarity grading. Most imperfections in gem quality diamonds are not visible to the naked eye. For this reason clarity is graded under 10-x magnification. Grades correspond to the International Clarity Grading Scale.

Click here to view clarity diagram chart from DCLA website.

The cut of a Diamond is the only property, which is totally dependent on man. Cut refers not only to the shape and style of the diamond, but its proportions, symmetry, and finish or “make”. Proportions and angles influence the internal reflection of light as well as the dispersion of light leaving the diamond. This determines the brilliance and fire (brightness and sparkle) of the diamond, and ultimately it’s perceived beauty.

Cut therefore, is actually one of the most important aspects to consider when choosing your diamond. A diamond can be cut for maximum weight recovery or maximum brilliance and beauty. Invariably one is traded of at the expense of the other. Although poor makes sell at a discount in the trade, realistically the unsuspecting retail customer will pay the same price regardless of make.

For Round brilliants there are several so called “optimum” cuts all with slightly different proportions. These are based on scientific formulas to maximize the return of light from the diamond. Ultimately taste and preference for the overall appearance determine the buyer’s choice.

Click here to view animated diagrams that explain the cut of a diamond.

Some common diamond shapes are pictured below:

The information featured on this page was sourced with permission from DCLA ( Diamond Certification Laboratory of Australia)