A rough diamond becomes a polished gem in a set sequence: the cutter maps the crystal, divides it, rounds the outline, places the main facets and then polishes the rest. Diamond is the hardest natural material, so only diamond can cut diamond, either as powder on a spinning wheel or as the blade coating on a saw. Lasers now handle much of the dividing and rounding. Every step involves a trade between keeping weight and making the stone look its best.
Planning the rough
Planning decides more about the finished diamond than any other step. A rough crystal can often yield one large stone, two medium stones or several small ones, and each option changes the total value.
Modern workshops place the rough in an optical scanner that builds a three-dimensional model of its outer shape. Separate inclusion-mapping systems locate internal features such as crystals and fractures. Planning software then tests thousands of possible cut layouts and estimates the weight, clarity and likely value of each. A skilled planner reviews the suggestions, because software cannot judge every feature, such as how a dark inclusion will reflect through the finished facets.
Before scanners, planners worked with a loupe and long experience. They studied the crystal's shape, its natural faces and the direction of its grain, then drew lines on the surface in ink to show where to divide it. Marking with ink still happens on some stones, though laser systems often record the plan digitally.
The crystal's shape guides the plan. A well-formed octahedron, the most common gem diamond shape, is often sawn across its middle to produce two round brilliants. A flat twinned crystal called a macle suits a triangular or heart shape. Irregular rough may become a fancy shape that keeps more weight than a round would. The crystal forms and twinning page covers these shapes in more depth.
Cleaving and sawing
Diamond has perfect cleavage in four directions parallel to the faces of an octahedron. A cutter who knows those planes can split a stone along them with one sharp blow. Traditional cleaving used a groove scratched into the surface with another diamond, then a steel blade set into the groove and struck with a rod or mallet. A clean blow splits the crystal neatly. A misjudged one can shatter it. The cleavage in gemstones page explains why some minerals split this way.
Sawing cuts across directions where cleaving is impossible. For most of the 20th century, workshops used thin phosphor bronze discs charged with diamond powder and oil. A single sawing cut through a one-carat stone could take hours, and larger stones took days.
Lasers changed this stage. A laser saw burns a narrow path through the crystal in any direction, ignoring the grain, and loses less material. Laser cutting also lets a planner divide rough into shapes that a blade or cleaving blow could not produce.
Bruting
Bruting, also called girdling, gives a round diamond its circular outline. The traditional method sets the stone spinning on a lathe while the cutter presses a second diamond, fixed to a long handle, against it. The two grind each other into rounded shapes, and the dust is collected for later use as polishing powder.
The edge left by bruting becomes the girdle, the narrow band around the widest part of the stone. Many bruted girdles look slightly frosted. Some cutters later polish or facet the girdle.
Automated bruting machines and laser girdling now do much of this work, especially for small stones. Fancy shapes such as pears and ovals are outlined on the polishing wheel or by laser instead of on a lathe.
Blocking and brillianteering
A round brilliant gets its facets in two stages, often done by different specialists.
Blocking places the first 17 or 18 facets: the table, the eight bezel facets on the crown, the eight pavilion main facets and sometimes a culet at the bottom point. The angles set at this stage largely decide how well the stone will handle light. A blocker who gets the pavilion angle wrong produces a stone that leaks light or looks dark in the center.
Brillianteering adds the remaining 40 facets: eight star facets around the table, 16 upper girdle facets and 16 lower girdle facets. These smaller facets break up reflections into the sparkle a viewer sees as scintillation. The round brilliant cut page describes every facet and the angles that work best.
Polishing on the scaife
Facets are ground and polished on a scaife, a cast iron wheel coated with a paste of diamond powder and oil. The wheel spins at thousands of revolutions per minute. The diamond sits in a holder called a dop or tang, which lets the polisher set the angle and press the facet against the wheel.
Diamond is harder in some directions than in others. Polishers say a stone has "grain," and a facet polished in the wrong direction barely changes, while the same facet turned a few degrees cuts quickly. Finding the right direction for each facet is part of the polisher's skill. Hardness on the Mohs scale is a single number, but this directional variation is one reason diamond is so slow to work.
Automated polishing heads with computer-set angles are common in large factories, particularly for small diamonds. Hand polishing remains standard for large and unusual stones.
Yield versus beauty
Cutting a round brilliant often removes around half of the rough weight or more. The cutter has to decide how much weight to give up for better proportions.
Weight thresholds push this decision. A stone that finishes at 1.00 carat sells for noticeably more per carat than one at 0.97 carat, so a cutter may keep extra weight in a thick girdle or a deep pavilion to stay above the line. The carat weight guide explains these "magic sizes," and how diamonds are priced covers the wider effect on value.
Common compromises include:
- A thick or very thick girdle that adds weight without adding size face-up
- A deep pavilion that keeps weight but darkens the center of the stone
- A large table on a shallow crown that makes the stone look bigger but shows less fire
- Leaving a small natural (part of the original crystal skin) on the girdle to save weight
Inclusions add another layer. A cutter may move the table to push a dark crystal toward the edge, where facets hide it, or sacrifice weight to remove it entirely and gain a clarity grade. Each choice shows up later on the grading report.
Where cutting happens
Most of the world's small and medium diamonds have been cut in India since the late 20th century, with Surat in Gujarat as the largest center. Large and high-value stones are often handled in Antwerp, Tel Aviv and New York. The history of diamond cutting traces how the work moved between cities over six centuries, and modern gem cutting technology covers scanners, lasers and automated machines across all gem types.
Frequently asked questions
How long does it take to cut a diamond?
A small diamond can go from rough to polished in hours with modern machines. A large or difficult stone can take weeks or months, mostly in planning and careful polishing.
Can a diamond break while it is being cut?
Yes. Cleaving along the wrong plane, internal stress or a fracture reaching the surface can split a stone during cutting. Scanning and laser sawing have reduced the risk but not removed it.
Why does cutting waste so much of the rough?
The ideal shape of a round brilliant rarely matches the shape of a natural crystal. Removing material to get correct angles and a symmetrical outline usually costs around half the weight.
What is the dust from cutting used for?
Diamond powder from bruting and sawing is collected and mixed into the paste used to polish other diamonds on the scaife.