A rough diamond looks nothing like the stone in a ring. It is dull, irregularly shaped, and often coated in a grey skin that gives no hint of the brilliance inside. The transformation from rough crystal to finished gem is one of the most exacting processes in any craft — part science, part intuition, and entirely unforgiving. A single miscalculation can shatter a stone worth tens of thousands of dollars. A perfect cut can make a modest diamond extraordinary.
The art of diamond cutting has been practised for over five hundred years, but the craft today would be barely recognisable to a fifteenth-century cutter. Technology has revolutionised every stage, from planning to polishing. What has not changed is the fundamental challenge: you must remove material to reveal beauty, and every gram you remove is gone forever.
Planning the Cut
Before a cutter touches the stone, the rough must be studied. This is the most consequential stage of the entire process — decisions made here determine everything that follows.
Reading the Rough
Every rough diamond is unique. Its crystal structure, inclusions, internal stresses, and external shape all dictate what the finished stone can become. A clean octahedral crystal — eight-sided, symmetrical — is the cutter's ideal. It naturally lends itself to a round brilliant with good weight retention. But most rough is not ideal. Macles (twinned crystals), flat or elongated shapes, and stones with significant inclusions all demand creative solutions.
The cutter's first task is to identify the stone's grain — the direction of its crystal lattice. Diamond is hardest perpendicular to its grain and can be cleaved along it. Understanding the grain determines where the stone can be split, where it can be polished efficiently, and where it will resist the wheel.
3D Scanning and Mapping
Modern planning begins with a 3D scan. Systems such as Sarine's DiaMension and Helium Polished use lasers to create a precise digital model of the rough crystal, capturing its external geometry and, in advanced systems, its internal features — inclusions, strain lines, and fluorescence zones.
Software then calculates every viable cutting option: which finished shapes are possible, what the weight retention will be, where inclusions will fall relative to the polished stone's facets, and what grade the finished diamond is likely to achieve. The cutter can rotate, section, and evaluate dozens of scenarios before making a single physical cut.
This is not mere automation. The software proposes; the cutter decides. An experienced planner knows that a computer model does not capture everything — subtle colour zoning, the way a particular inclusion might reflect between facets, the commercial reality that a slightly smaller stone of better colour may be worth more than a larger one. The best planning is a conversation between technology and expertise.
Cleaving and Sawing
Once the plan is set, the rough diamond must be divided. There are two methods, and the choice between them depends on the stone's structure and the cut being pursued.
Cleaving
Cleaving splits a diamond along one of its four natural cleavage planes — directions in the crystal lattice where atomic bonds are weakest. The technique is ancient. A groove is scored into the diamond's surface with another diamond, and a precisely placed blade strike splits the stone along the plane.
Cleaving is dramatic and irreversible. It is used when the rough crystal's natural geometry makes a clean split possible — for instance, separating a large octahedron into two pieces, each destined to become a separate gem. The famous Cullinan Diamond, the largest gem-quality rough ever found, was cleaved into nine major stones by Joseph Asscher in 1908. He reportedly studied the stone for months before making the first cut.
Today, cleaving is less common in commercial production. Laser technology has made sawing the preferred method for most divisions. But for certain crystal configurations, particularly macles, cleaving remains the most effective approach.
Laser Sawing
Laser sawing uses a focused beam — typically a Nd:YAG or fibre laser — to cut through the diamond along any desired plane, not just the natural cleavage directions. The laser vaporises a thin line of carbon, slicing through the stone with minimal material loss (the kerf is typically 0.1 to 0.3 millimetres).
The advantages are significant. Laser sawing is more precise, more predictable, and allows cuts that would be impossible by cleaving. It can follow curved paths, cut through inclusions that would cause a cleave to propagate unpredictably, and work on irregularly shaped rough where no clean cleavage plane exists.
The process is slow — cutting through a one-carat rough can take hours — but the control it provides makes the time worthwhile. Most modern cutting facilities rely on laser sawing for the majority of their work.
Bruting: Shaping the Outline
After the rough is divided, the next step is bruting — also called girdling. This gives the diamond its outline shape.
For a round brilliant, the most common cut, bruting means grinding the stone into a circle. Traditionally, this is done by mounting two diamonds on spinning lathes and pressing them against each other. Diamond is the only material hard enough to shape diamond, and the friction between the two stones gradually rounds both into the desired profile.
The girdle — the narrow band around the diamond's widest point — is formed during bruting. Its thickness and consistency matter: a girdle that is too thin risks chipping during setting; one that is too thick adds unnecessary weight without contributing to the stone's appearance. Skilled bruters aim for a consistent, medium girdle that balances durability with proportion.
Modern bruting increasingly uses laser technology as well. Laser bruting allows non-round shapes — ovals, cushions, pears — to be formed with greater precision than manual methods permit. For fancy shapes, where outline symmetry directly affects light performance and visual appeal, this precision is particularly valuable.
Faceting: Where the Light Begins
Faceting is the heart of diamond cutting. It is the stage that determines how the finished stone interacts with light — its brilliance, fire, and scintillation. Every facet is a tiny window, and each must be placed at the correct angle, with the correct proportions, relative to every other facet.
The Polishing Wheel
A diamond is faceted on a cast-iron disc called a scaife (in Antwerp) or tang (in other traditions), spinning at approximately 3,000 to 4,000 revolutions per minute. The disc is charged with a paste of diamond powder suspended in olive oil — again, only diamond can polish diamond. The cutter holds the stone in a mechanical arm called a dop or tang, positioning it against the spinning disc at precise angles.
Each facet is polished individually. A standard round brilliant cut has 57 or 58 facets — 33 on the crown (the top half, above the girdle) and 24 or 25 on the pavilion (the bottom half, below the girdle). The table, the large flat facet on top, is polished first. Then the crown facets: kite-shaped bezel facets, triangular star facets, and upper girdle facets. The stone is then turned over and the pavilion facets are placed: lower girdle facets and the main pavilion facets that are primarily responsible for returning light to the viewer's eye.
Angles and Proportions
The science behind faceting is geometrical optics. Light enters the diamond through the crown, bends as it passes from air into the denser medium of diamond (which has a refractive index of 2.417), bounces off the pavilion facets, and exits back through the crown. For this to work — for the light to return to the viewer rather than leaking out the bottom or sides — the pavilion facets must be angled correctly.
The GIA's research into light performance has established that a crown angle of approximately 34 to 35 degrees and a pavilion angle of approximately 40.6 to 41 degrees, combined with a table size of 54 to 57 percent of the girdle diameter, produce the strongest balance of brilliance (white light return), fire (spectral dispersion), and scintillation (the pattern of light and dark as the stone moves). (Diamond Anatomy and Light Interaction)
But these are guidelines, not rules. Master cutters know that slight adjustments to these angles — a half-degree here, a percentage point of table size there — can optimise a particular rough stone's performance. Two diamonds with identical grades on a grading report can look quite different in person, and the subtlety of the faceting is often why.
Fancy Shape Faceting
Round brilliants follow a well-documented formula. Fancy shapes — princess, emerald, oval, marquise, pear, cushion, radiant, Asscher — each have their own faceting conventions, but with more room for variation. An emerald cut has long, open step facets that emphasise clarity over fire. A cushion cut blends brilliant and step elements. Each shape demands a different set of skills and a different eye.
The lack of standardised proportions for fancy shapes means the cutter's judgement matters even more. What makes an oval "well-cut" is less codified than for a round brilliant, and the best fancy-shape cutters develop an intuitive sense for the proportions that produce the most appealing light performance.
Quality Control
The finished diamond must pass inspection before it enters the market. Quality control checks every element of the cut.
Proportion Analysis
Digital proportion analysers measure every angle, every facet length, and every symmetry element of the polished stone. The results are compared against the target specifications from the planning stage and against industry grading standards. For stones destined for GIA grading, proportion analysis predicts what cut grade the diamond will receive.
Symmetry and Polish
Symmetry refers to how precisely the facets align with each other — whether the table is centred, whether opposing facets are the same shape and size, whether the culet (the tiny facet at the very bottom, if present) sits at the exact centre of the pavilion. Polish refers to the smoothness of each facet surface: scratches, burn marks, or polishing lines left by the wheel all affect the grade.
Both symmetry and polish are graded on a scale from Excellent to Poor. Stones graded Excellent in both categories — "triple excellent" when combined with an Excellent cut grade — command a premium in the market, particularly in the Japanese and American retail sectors.
The Human Eye
Despite all the technology, final quality assessment still involves an experienced human looking at the diamond through a loupe and, critically, with the unaided eye. Does it flash with life? Is the scintillation pattern even and appealing? Does it face up white, or does the body colour concentrate in certain areas? These are questions that instruments inform but cannot conclusively answer.
Modern Technology vs Traditional Craft
The diamond cutting workshop of 1950 and the one of today share the same fundamental goal, but the tools have changed profoundly.
What Technology Has Changed
- Planning: 3D scanning and AI-driven optimisation software have replaced hours of hand-drawn study with precise digital models. A planner can evaluate more options in minutes than a previous generation could in days.
- Sawing: Lasers have replaced most mechanical saws and many cleaving operations. The precision and flexibility are transformative.
- Bruting: Automated bruting machines maintain tighter tolerances than hand-guided methods, especially for round stones.
- Faceting: Computer-controlled faceting machines (such as those by Sarine, OGI Systems, and Lexus) can position facets with sub-degree accuracy, and some can fully automate the cutting of smaller stones.
- Inspection: Digital scanners, light performance imaging (ASET, Hearts and Arrows viewers, Idealscope), and automated grading systems provide detailed, reproducible assessments.
What Technology Has Not Changed
The craft still requires human judgement at every critical juncture. Software can calculate the optimal theoretical cut, but only an experienced planner can weigh the commercial trade-offs — the interplay of weight, colour, clarity, and market demand — that determine the right cut in practice. Automated faceting machines cut small stones efficiently, but large, high-value diamonds are still cut by individual craftspeople who feel the stone respond to the wheel.
The best cutting houses combine both. Technology handles the routine and the measurement. Human expertise handles the decisions and the artistry.
The Yield-vs-Beauty Trade-Off
This is the central tension of diamond cutting, and it has been for centuries.
Yield is the percentage of the rough diamond's weight retained in the polished stone. A higher yield means a bigger finished diamond, which generally means a higher price. Cutters — and the businesses that employ them — have a financial incentive to maximise yield.
Beauty is the optical performance of the finished stone. Achieving the ideal proportions for light return often requires removing more material than a yield-maximising cut would. The pavilion might need to be slightly deeper, the crown slightly steeper, the girdle trimmed tighter — all of which reduce the final carat weight.
Where the Tension Plays Out
The trade-off is sharpest at "magic sizes" — weight thresholds where price per carat jumps significantly. A diamond that weighs 1.00 carat is worth meaningfully more per carat than one that weighs 0.97 carats. A cutter facing this threshold must decide: cut to ideal proportions and land at 0.97, or adjust the angles slightly to preserve that extra weight and cross the one-carat mark?
The market has, for years, rewarded weight. But this is shifting. The GIA's cut grading system, introduced for round brilliants in 2006, gave consumers a tool to evaluate optical performance independently of weight. Stones graded Excellent for cut increasingly command premiums over heavier stones with lower cut grades. The message is reaching the market: a well-cut 0.95-carat diamond can outperform a poorly proportioned 1.05-carat stone in every way that matters to the eye.
At Arete, this is a principle we take seriously. We would rather show you a diamond that is alive with light than one that is simply large. (The 4Cs Overview)
Summary
- Planning is the most consequential stage. 3D scanning and software model the rough, but the human planner makes the final call on how to cut it.
- Cleaving splits along natural crystal planes; laser sawing cuts in any direction. Lasers have become the dominant method for their precision and versatility.
- Bruting gives the diamond its outline shape, with two diamonds ground against each other to form a round or a laser-guided profile for fancy shapes.
- Faceting determines light performance. The angle and placement of each facet controls brilliance, fire, and scintillation — and a round brilliant has 57 or 58 of them.
- Quality control combines instruments and the human eye. Digital proportion analysers measure; experienced graders evaluate the overall result.
- Technology has transformed the tools, not the decisions. AI plans, lasers cut, and machines polish — but the craft still depends on human expertise at every critical moment.
- Yield and beauty are in tension. The best cutters — and the best jewellers — prioritise light performance over raw carat weight.
Related reading: The Modern Diamond Pipeline | Why Antwerp Matters | Pricing Basics | A Short History of Diamonds in Jewellery