Sorting process

• Diamonds weighing more than 10.80 carats (¢) are classed as Large Stones or Special Stones. Each stone is assessed by hand by sorting experts to determine its value.
• Diamonds weighing between 0.66 carats and 10.79 carats are divided into a number of weight categories using bespoke sorting technology designed by the Diamond Trading Company (DTC).
• Diamonds weighing less than 0.66ct are divided into a further six categories using sieve plates.

The second step in the sorting process is model.

‘Model’ is the term used by the DTC to refer to the shape of the stone. The shape of the rough diamond often dictates the final shape and size of the polished stone.

Rough diamonds fall into two main model categories, Sawable and Makeable.

Sawable stones are often octahedral or dodecahedral in shape.  The rough diamond is sawn prior to being polished.  This will produce two polished stones.

Makeable diamonds tend to be half broken octahedrons or dodecahedrons or flatter whole models.  They produce one polished stone and usually require more work than Sawables to create the final polished stone.  They also yield less polished from the rough.  These are then subdivided depending on the shape of the final polished from the rough stone.

The third step is to decide on the quality. 

Quality is a measure of the impurities, or inclusions, found within the stone.  The number, position and type of inclusions affects the value of the diamond because they impact the way light travels through the stone, which may reduce the fire and brilliance of the polished diamond.

A clean stone also holds greater value because of its rarity.

There are two broad categories in quality sorting - “gem” and “near gem”.

When grading gem quality rough diamonds we look for inclusion(s):

• How many are there?
• Where are they located?
• How are they spread throughout the stone?
• How will they impact the look of the eventual polished stone?

When grading near gem quality rough diamonds we look at what percentage of the diamond is clean. The more inclusions in the stone, the lower the quality grade will be. 

Near gem quality stones will either be polished for the jewellery market or, in the lower grades, divided into smaller cleaner stones or used for industrial purposes such as in drills, saws and scalpels.

The final step is for the sorter to decide on the colour.

Most commonly, the colours of diamonds fall into categories from colourless to yellow and brown, although some rarer colours such as blue, orange, pink and red do appear from time to time.  Diamonds described as ‘white’ are actually colourless.

Although much of the DTC’s sorting process is carried out by hand using traditional handloupes and headloupes, since 1972 the DTC has used technology to assist with the sorting of smaller diamonds. 

The DTC’s Research & Development centre in the United Kingdom works closely with the company’s sorting and valuing offices to produce machines that provide unparalleled operating efficiency, flexibility and consistency for the DTC’s diamond sorting process.