Transformation of a Natural Coloured Rough Diamond to a Polished Diamond

Diamonds, in their initial rough state, are hardly impressive. There are none of the brilliant flashes of light… yet. It is only after the diamond undergoes polishing that its real brilliance – and value – is revealed. Helping the transformation from a natural coloured rough diamond to a polished diamond is an art form that requires high levels of skill. And the only way to appreciate a diamond’s transformation is to follow it from the mine to the jeweller and appreciate how each brilliant gem is transformed.

Right out of the ground, diamonds are naturally opaque and usually coloured. Only about 25% of the raw stones found are without some faint colour. About 30% of raw stones unearthed show a light shade of colour, such as violet, yellow, or brown. These are known as ‘off-colour’ stones. Now, for the remaining 45%, these stones are, more or less, deeply-coloured and consequently of no jewellery value, although still useful for diamond cutting and polishing. 

How is a natural coloured rough diamond transformed to a polished diamond? The whole process of cutting and polishing diamonds can be broken down into four main steps:

  • planning 
  • cleaving
  • bruting
  • polishing 

In the planning stages, the cutter determines the best possible shapes for each rough stone, with minimising waste and maximising yield as the goals. Typically, a rough stone is mapped using a Sarin machine which generates accurate measurements. The data that’s gathered is used to formulate 3D models that will help the cutter optimise the rough stone. 

The cleaving process is where the splitting of the rough stone into pieces happens. This allows the cutter to work on the pieces separately and also to fully utilise the rough diamond. Mechanical sawing may also happen at this stage for any oddly-shaped roughs. The sawing process can be done with the use of non-contact cutting tools like lasers. 

After the diamond is split, bruting is done to make the separated rough stones round. This process is also referred to as girdling. In bruting, two diamonds are placed on a spinning axle across each other. They are spun in opposite directions and grind against each other to create a rough girdle finish.

But let’s grind a little into the finer details for a bit first…

Transformation of a Natural Coloured Rough Diamond to a Polished Diamond - Diamond

Sorting and Planning Rough Diamonds

A diamond begins its journey with the sorter. The sorter is an expert on diamonds with a trained eye as they need to be able to detect the slightest variations in the colour of diamonds and to find flaws in the stones. This expert diamond appraiser sits with a pile of rough stones, judging each and assigning them to their appropriate heaps. 

The first factor to consider in diamond sorting is the stones’ adaptability for cutting. The best heap includes stones considered as ‘close goods’. This grade is comprised of complete, flawless crystals from which brilliants of fair sizes can be derived or, to use the trade term, ‘made’. To be considered good enough for the ‘close goods’ heap, a rough stone would typically have to have eight faces and be triangular in shape. This heap of ‘close goods’ is then further sorted into eight grades, ranging from blue-white (finest quality stones), to yellow and brown (off-coloured and unfit for gems).

Cleaving Rough Diamonds

In the cleaving (or splitting) phase, the surface irregularities, along with any superficial flaws or dark patches, are split away from the stone, with breaks naturally occurring along smooth, even surfaces parallel to the natural faces of the crystal. 

To accomplish this, the diamond is firmly cemented to the end of a wooden stick. Then, with the sharp corner of a diamond fragment, a deep scratch is made on the surface of the diamond. A knife’s edge is then held at a right angle upon the scratch, and with a light tool, a sharp blow on the back of the knife’s edge is applied to remove the undesirable flake, leaving the diamond’s surface smooth and bright. 

For larger stones, mechanical sawing may be necessary. This is accomplished with a thin disk made of bronze (about 10cms in diameter) revolving very rapidly, its edge charged with diamond dust at the beginning of the sawing. As the saw bites into the stone, it is recharged with the diamond dust coming from the sawing. It takes several hours for this small saw to eat its way through half an inch of diamond, but the finished product is so valuable that the time this part of the process takes is totally worth it in the end.

Bruting Rough Diamonds

After the diamond has been split, the rough shaping of the diamond is done through a  phase called ‘bruting’. This process involves wearing away the corners of a diamond by rubbing one stone against another. In earlier times, bruting was a strictly manual process, with the two diamonds being mounted on hand-held sticks. Nowadays, this process is accomplished by a rapidly twirling spindle. 

Once a stone has passed the critical eye of the sorter and has ended up in one of the higher grades, it is then weighed, wrapped up in a parcel with others of its kind, assigned a price-per-carat, and is sold to a diamond dealer. This is when a stone finally finds its way to the diamond polisher. Here, the facets of the diamond which give it the brilliancy and sparkle produced through a process referred to as polishing. The rough stone is placed on a rotating arm and a spinning wheel is used to polish it. This creates the smooth and reflective facets on the diamond.


Polishing Rough Diamonds

During polishing, the stone is held in a small metal cup at the end of a long stem (called a ‘dop’) A solder made of one part tin and three parts lead is placed in the dop and heated until soft. The diamond is then set in the solder with the portion of the stone on which the desired facet is to be cut facing upwards. When the diamond has been properly set on the dop, it is then plunged in cold water to cool and harden the solder. In this part of the process, less aristocratic stones will likely shatter to pieces – only a diamond can withstand the conditions of this process. Diamonds are known to have high heat conductivity, thus drastic changes in temperature never cause even the slightest flaw.

Going into detail, this polishing phase of a diamond can be further divided into two steps: blocking and brillianteering. In blocking, the following facets are added: 8 pavilion mains, 8 crowns, a culet and a table facet, to produce a single-cut stone. In brillianteering, the remaining facets are added, bringing it all to a total of 58 facets. The importance of this step is to create a ‘canvass’ for the next stage. It takes several hours to cut each facet, and then the stone is re-adjusted for another facet. This goes on until all of the 58 dainty facets in which lies the brilliancy of the diamond are produced. It can be said that the brillianteer holds the greatest responsibility as they finish the job, bringing the fire and brilliance of the diamond to completion.

Argyle Diamond Investments Pty Ltd are Australia’s largest supplier of Australian certified pink diamonds and have exclusive use of the Australian Pink Diamond Analytics program. This program is a data driven decision maker that tracks and statistically analyses the rarity, financial growth and value of all investment grade pink and blue diamonds. With this valuable information, Argyle Diamond Investments Pty Ltd can offer all clients an investment grade pink from $5,000AUD up. But, the highest growth diamonds, are the larger, stronger colour pinks starting from around $20,000AUD plus.


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