How Gemstones are Cut
See, step by step, how a gem takes shape
When gemstones are first mined, they look like pretty pebbles. It takes the skill of a gem cutter to shape a gem to reflect light back as brilliance and to polish its surfaces into mirrors that dazzle.
Cutting a gemstone is a slow process of grinding down some of the world’s hardest substances into geometrically precise prisms. It takes dedication and skill to accurately execute each step in the process of cutting and polishing so that each facet is perfectly aligned with the next.
This 4.75-carat rough blue sapphire was mined in Tanzania. It was probably panned from the gravels of a river bottom. Because it is worn by the water, you can already see its color: sometimes rough is rougher.
This stone has a nice symmetrical shape so it is ideally suited for cutting a round gemstone. For extra brilliance, we will be cutting a Portuguese round, which has more facets than a standard round brilliant. We will show you step-by-step how a precious pebble becomes a faceted gemstone
Step 1: Preforming Arts
The first step in cutting is performing: roughly shaping the gem so that it has the profile of the gem shape we will be cutting. Choosing the shape and orientation of the final stone is an art. The cutter is looking to select the largest possible size and shape that the random shape of the rough will accommodate. Sometimes a crack or flaw in the crystal requires some creative thinking. It may be necessary to saw a piece of rough into two to get the best yield.
Preforming is usually done by hand with fairly coarse diamond abrasive that cuts quickly. The idea in this step is to grind away any unneeded material quickly to save time later. Here is the same sapphire after performing: it’s roughly round and has a flat spot in the middle that will eventually be the table of the stone.
Step 2: The First Lap
Faceting machines vary, but they all have a rotating wheel that actually does the grinding, which is called the lap. The lap has a replaceable surface that is impregnated with bits of diamond. Diamond, while the most expensive of abrasives, cuts crisp, beautifully polished facets so quality cutters use it for cutting all gem varieties.
The friction inherent in grinding and polishing a gem creates a lot of heat. In fact it can literally melt the surface of the facet. To keep things cool, the lap is lubricated by running water.
In addition to a water-cooled spinning lap, all faceting machines have an arm that holds the gemstone to be cut on a stick called the dop. This arm must be able to be precisely positioned at a fixed angle and rotation to put each facet in the right place. This can be done as simply as using holed drilled into a board, which is called a jam-peg cutting machine. The Facetron machine we show here uses an index gear.
Step 3: Getting on the Stick
To begin cutting, the sapphire is attached to the dop stick with a big glob of wax on the table. A hollow cone is used to center the stone perfectly on the dop so it is ready for faceting.
First, the arm of the faceting machine is set at 90 degrees and the gem is turned smoothly against the lap making it perfectly round.
Next, it's time to begin cutting the pavilion facets that meet in a point at the bottom of the stone, called the culet. First each of the 16 rough facets are cut to establish the depth of the pavilion which is the bottom half of the stone. The angle of these facets is calculated according to the refractive index of the gem material being cut; how much that gem bends light. A cutter needs to know how gems handle light in order to maximize brilliance.
After these main facets are cut, the cutter switches to a finer grit diamond lap to begin the final cutting of smaller facets and the final polishing. For the fine cutting of sapphire, the cutter uses fine diamond paste on an aluminum lap. The aluminum transfers the heat out of the sapphire a lot better than other polishing laps, and lets the cutter polish harder without melting the wax that holds the stone on the dop. Depending on the type of gem, polishing may be done with a tin lap and corundum powder or cerium oxide polish. Sometimes oil is used as a lubricant instead of water.
Step 4: Crown Molding
After the pavilion is polished, the stone is removed from the dop. It’s reattached to a second dop with a pointed impression with room for the pavilion with wax and a bit of crazy glue to allow the top half of the stone to be faceted.
The new dop with the stone on goes back onto the arm of the machine to start cutting the facets of the crown. Notice you can still see a little wax in the middle of the stone from where the original dop was stuck onto it.
The cutter then polishes all three rows of 16 facets that make up the crown of the stone. The sapphire is now beginning to sparkle like a faceted gem.
Step 5: Table Manners
The next step is to cut and polish the table, the flat spot on the top of the stone. On this machine, cutting and polishing the table requires the use of an adapter that is plugged into the arm of the machine. Because sapphire is so hard and the table is relatively large, it takes a long time to cut and polish. A large gem that is carefully cut for precision and maximum yield may take up to 6 hours to cut, from start to finish.
The last step is to polish the "girdle" or edge of the stone. A lot of cutters don't bother, but a polished girdle adds extra brilliance and also makes the gem more attractive when the stone is set in a mounting where the girdle shows.
Step 6: The Reveal
It's done! After washing away the grit and residue, the brilliance of the faceted gem is revealed. The final faceted sapphire is less than half the size of the original piece of rough at 2.17 carats. But the humble pebble has become a gem.
How can you tell when a gem like this is well cut? The evidence is easy to see: a well-cut gem reflects light back to your eye evenly over its entire surface. No dark shadows or sleepy spots distract you. You shouldn’t be able to see though the table: clumsily cut gems have a window, a hole in the brilliance in the center of the gem. A well-cut gem beams light at you and sparkles when it is moved. There are no short cuts; quality cutting means that every facet has been positioned precisely and symmetrically to reflect back light as brilliance. Meticulous polish can give a gem exceptional luster.
The style of cutting also affects performance. Because some cuts have more facets, they have smaller confetti-like patterns of reflection, which some people prefer. The outline or shape of the cut is also a matter of preference. See our Gemstone Cut & Shape Glossary for a visual reference to the wide variety of cuts available today.
