You may be wondering, what’s the point of this tutorial? After all, we are diamond prosumers not diamond cutters. Well, not only does learning a bit about the process of diamond cutting allow you to appreciate the value of your diamond more, it can also help you understand the cutter’s thinking process so you can better understand their decisions. This helps us make better buying decisions.
Before a diamond is cut, a lot of thought goes into how to best utilize the rough diamond. I won’t be discussing the long process of the planning stage in this tutorial, as most of the value there is for the cutter. What I want to focus on are the three main cutting procedures that diamond prosumers should know about:
Cleaving is a technical term for the cutting up of the rough. The term is familiar to those of us who are engineers referring to cleavage planes where the material is weaker. It is often said that because diamond is so hard, only another diamond can cut it. Now this isn’t strictly accurate because hardness only refers to a material’s resistance to being scratched. Engineers also know that the harder a material is, the more brittle it becomes. So although a diamond is the hardest material in the world, it is also extremely brittle. This means that it is easier to cut a diamond along its cleavage planes which are along the material’s grain boundaries. If you want to understand graining, just think about the grains in wood or how much weaker corrugated cardboard when it is parallel to its corrugated lines that it can be rolled up in one direction but is very stiff in the other.
Diamond rough can be found in dodecahedral (12-face), octahedral (8-face), cubic (6-face), and macle (twin) crystals and each have their own particular cleavage directions so that diamonds are cut differently depending on which shape the rough is. It is important to a cutter which shape the rough is because different shape diamonds will be cut depending on the shape of the rough in order to maximise their rough yield.
The ideal rough shape is octahedral because polished round brilliant diamonds have 8-sides therefore this type of rough generally produces the largest rough yield. That’s as technical as I want to get because it really is just for the sake of knowing, unless you suddenly find yourself in the market for gem quality diamond rough.
Now the process of bruting is where it is true that one diamond is used to cut another. Traditionally, it is a turning process using a type of lathe where the gem quality diamond is ground against a much lower quality industrial grade diamond in order to make its edges round, which becomes the diamond’s girdle. If the girdle is left in this form, then it is what is known as a ‘bruted girdle’. Better quality diamonds have a girdle that is faceted or polished. Nowadays, bruting can be done using a laser cutting machine.
Now polishing is the part that I am most interested in. There are two parts to polishing. The first part is known as blocking, which is the cutting of the crown and pavilion facets, and in doing so forming the table, girdle, and culet. The process is relatively straight-forwards and for ‘single cuts’, also known as ‘eight cuts’, this is the end of the cutting process.
For the traditional 58-facet modern round brilliant cut, also known as a ‘full cut’, the single cut diamond is sent to a brillianteer. This specialist cutter is appropriately named after the brillianteering process, where the minor facets are cut. Here the brillianteer makes the decisions regarding how to make the diamond appear its best at the same time retaining weight to ensure that a diamond makes it into a certain carat range.
Where is your diamond hiding its weight?
The cutter’s decision making in the blocking process are:
• How big should the table be?
• How steep should the crown angles be?
• How steep should the pavilion angles be?
• How thick to make the girdle?
The cutter’s decision making in the brilianteering process are:
• How long should the lower girdles be?
• Should the girdle be painted?
• How big should the star facets be?
Each of the above questions is influenced by the cutter’s intention to retain weight in order to produce a higher rough yield. We start with the girdle; a thick girdle tells us that the cutter needed to add a significant amount of weight. The girdle is probably the first choice because you can safely add over 1% of the diamond’s weight, by making a slightly thick girdle, without affecting its GIA cut grade.
A slightly thick girdle is a warning sign because the chance of other weight retaining techniques used becomes higher. The infamous ‘steep-deep’ diamond is another way to retain weight but is detrimental to the diamond’s appearance and won’t make it to a GIA ex cut grade. But there are also slightly steep-deeps, for example a 58% table with a 35-degree crown angle paired with a 41-degree pavilion angle. These diamonds look perfectly good and fall squarely within the GIA excellent cut grade. Yet it allows the cutter to retain a fair amount of weight.
The decision on how to cut the lower girdles is straight-forwards if the cutter is cutting to H&A specifications. However, there are times where the lower girdles are used to manipulate the brightness of the diamond. The story of a diamond cut for weight retention continues for example, this time if the pavilion angle is shallow and the table is large. Longer lower girdles will help reduce some of the head shadow/body obstruction.
Significant crown-only painting of the girdles will make a diamond miss out on a GIA excellent cut grade. However, it is possible that it will make an AGS0 cut grade. It is not unreasonable for crown and pavilion painting to add up to 4% of the weight of a diamond. However, the technique is often used more modestly and slight crown-only painting can be used to retain a little bit of weight with perhaps only a slight decrease in contrast in the upper girdles.
In general, longer and steeper stars also mean steeper upper girdles and this combination helps weight retention. Note that painting of the upper girdles actually makes them shallower at the same time retaining weight and this seems to be a contradiction. But the reason this kind of painting helps weight retention despite having shallower upper girdle angles is because less material is removed to create the effect. Remember from the upper girdle tutorial that if we paint the girdle while keep the crown height the same, then the girdle thickness at the half facets becomes thicker; this is the weight retaining form of painting. But if instead we were to try to keep the girdle thickness the same as it would have been if the diamond weren’t painted, then in order to keep the crown angle the same, the crown height decreases so that crown-only painting actually loses weight rather than retaining it.
Lessons for Buying Blind
Although I do not not advocate buying blind, that is without idealscope and H&A images, I understand that some people may find that they are limited to what information is available to them in their diamond search. An understanding of the polishing process of a diamond can help us make better educated guesses when we have no other choice.
Now because the crown and pavilion facets are cut before the upper/lower girdles and the star facets, the following 3 facts must be true:
• The lower girdle angles are always steeper than the pavilion angles; and
• The upper girdle angles are always steeper than the crown angles; and
• The star facet angles are always shallower than the crown angles.
In general, the star facet angles in degrees are in the 20’s, the crown angles in the 30’s and the upper girdles in the 40’s. This helps the diamond achieve its scintillation because there is a clear distinction of how each facet handles light from any particular direction. The lower girdle and pavilion angles are more similar to each other and are both usually in the 40’s. This is because slight changes in angle makes a big difference to its depth.
This also explains why the lower girdles leak light before the pavilion facets. Logically, the problem is worse if we have steeper lower girdles. One thing we can learn is that we can avoid some leakage when we have steeper crowns by compensating with shallower lower girdles. In general, shorter lower girdles have steeper angles and a cutter can make them shallower by either lengthening them or by painting them.
Another thing we can learn from this is that longer and steeper stars mean even steeper upper girdles. How can we use this knowledge? Well if we are buying blind and the diamond has relatively longer and steeper stars, say 55%, you may be worried that the star percentage could really be 57%. The risk here is when you have steeper crowns, and the upper girdle facets are also steeper and we know that upper girdles 44 degrees or steeper will cause significant leakage. It would therefore make more sense to pair diamonds with steeper crowns and stars with longer and shallower lower girdles to compensate for the steeper upper girdles.
The combination of these effects suggests to me that to be safe, we should avoid combinations of 55% stars and 75% lower girdles in diamonds with slightly steeper crown angles when buying blind. It is possible that in diamonds with large variances, that there are a few of the lower girdles with angles that are too steep so that they are leaking light.
The opposite is true if you have shallower crown angles because now we are worried that shorter star facets will lead to overly shallow upper girdles so you will want to pair the shallower crown angle with relatively steeper stars to be safe. If you find yourself buying blind with a 34/41, then you could consider 55% stars and 80% lower girdles. But note that this combination will produce relatively larger hotspots and too much obstruction may make the diamond appear dark sometimes; having a steeper pavilion angle can help in minimising the obstruction problems. On the other hand, if you have a 35/40.6, then it will probably be safer to get a 50% star with 75% lower girdles in order to avoid any potential leakage of the lower girdles.
A final point is that if you wanted to optimise for brightness or fire within the TIC range, then I usually recommend a diamond that is tending towards brightness rather than fire (34/41) to be paired with 50% stars, which is the opposite to what the analysis above would suggest to be a safe match. Similarly, some people may wish to optimise for fire by choosing 55% stars with a 35/40.6. The point here is that optical symmetry and precision cutting is more important for those who wish to optimise further for their personal preferences regarding brightness or fire.