Gemstone Faceting Terms: A Glossary for Students
A glossary of frequently encountered gemstone faceting terms for the student gemologist. Our list covers gemology, gem cutting, and equipment.
19 Minute Read
A solvent used to dissolve dop wax or help release super glue.
Positioning the stone so that the crown break facets match up with the pavilion break facets at the girdle after transferring the stone.
Analog vs Digital Angle Scales
Some newer faceting machines have digital angle scales that allow users to adjust the quill to a 100th of a degree, which corresponds with modern gem designs produced in typical GemCad format. The average machine has a protractor or analog angle dial which reads to 1 degree, with a vernier scale which allows further adjustment to a 10th of a degree.
This can refer to one of the following:
- The direction of crystal growth in gem material, such as the A-axis or width, the B-axis or depth and the C-axis or length. In some tourmalines, the C-axis may be blocked, also referred to as "closed," which prevents light from passing through it.
- In a symmetrically cut stone, the axis refers to an imaginary line passing from the center of the culet or keel through to the center of the table, also known as the vertical axis.
- On a cutting design, the X-axis is an imaginary line that passes from left to right through the girdle; the Y-axis is an imaginary line that passes from front to back through the center of the stone at the girdle; and the Z-axis is an imaginary line that passes at right angles to the X and Y axis, usually corresponding to the vertical axis.
A cylindrical or carrot-shaped synthetic gem formed as the result of a flame fusion process, such as synthetic corundum and spinel. In the case of most synthetic corundum, manufacturers split the boule down the middle, lengthwise, to relieve internal stresses that result from the creation process. These split pieces are referred to as split boules.
A term that refers to the total amount of light return that results from internal and external light reflection. Factors that affect brilliance include the gem's refractive index, design proportions, polish, and the clarity of the material.
An instrument used to measure the dimensions of a stone.
CAM and CLAM
Pre-form methods. CAM (Center Point Angle Method) involves cutting 3 or 4 temporary pavilion facets to establish the culet of the stone, then cutting the same number of corresponding girdle facets to establish a basic pre-form. CLAM (Corner Locator Angle Method) involves cutting two pavilion facets that make a line from the culet to the corner of the pre-form.
An instrument used to measure the weight of a cut stone in carats. Some digital carat scales can also weigh in grams, pennyweight, ounces, troy ounces, and drams.
A polishing lap that can be charged with diamond slurry.
The process of impregnating a lap with grit.
A colloquial term that refers to the common technique for finessing a facet to meet where you want it. It doesn't denote an inappropriate action. Faceters most often "cheat" after cutting and polishing the table to make sure all the facets that should be touching the table are doing so. It involves re-cutting or re-polishing a facet at a slightly different angle than the design calls for, so that it meets the table or another facet where it should.
Cleavage and Fracture
Terms that refer to the way a stone breaks. If the stone breaks along a certain flat plane, this surface is called the cleavage plane. If the stone breaks along an uneven or irregular surface, this is called a fracture.
Different minerals and crystal structures can absorb different wavelengths of light. For most gems, color is principally the result of the wavelengths of light that aren't absorbed by the mineral or crystal structure. We see these unabsorbed wavelengths as color.
Allochromatic gems contain impurities that absorb portions of the light spectrum. For example, chromium in emeralds absorbs specific wavelengths of light, so what we ultimately see as unabsorbed light from emeralds is a green color. However, in rubies, chromium absorbs other portions of the light spectrum, so what we ultimately see as unabsorbed light from rubies is red. In both cases, the same impurity element works with specific minerals and yields different colors.
A phenomenon associated with a variety of gemstones. It may refer to one of the following situations:
- Some gems may change color under different light sources. They may appear one color under one type of light, such as incandescent, and another color under another type of light, such as sunlight or fluorescent light. For example, alexandrite appears blue-green in daylight and red in incandescent light.
- The color of some gems can be altered over time by exposure to sunlight or other forms of naturally occurring radiation. This is known as tenebrescence. Some gems, like spodumene, can even change color in darkness.
- Some types of colored stones can be heated or irradiated in laboratories to change their color. Tanzanite usually occurs naturally as brownish zoisite but turns blue when heated. Topaz, usually colorless or yellow-orange in nature, turns blue by exposing it to gamma radiation.
Under halogen or sunlight, this stillwellite crystal on matrix appears red. However, under fluorescent light, it turns yellow. Darai-Piez Massif, Alay Range of Tien-Shan' Mountains, Garm Region, Tajikistan. © Rob Lavinsky, www.iRocks.com. Used with permission.
This refers to all gemstones other than diamonds, regardless of their color. This term replaces the traditional distinction between "precious and semi-precious" stones.
A phenomenon that occurs in many types of gem rough, where the color isn't evenly dispersed throughout the body of the stone but occurs in pockets or layers instead. The stone can have zones of lighter, darker, or different colors.
Located above the girdle or at the top of a cut stone, the crown faces up and is in plain view when the gem is arranged in a setting.
The pavilion angle at which the light that enters a gem will be totally reflected, producing excellent sparkle and color. If the pavilion angle is cut below the critical angle for a gem, light will escape the gem and not return to the eye of the viewer. In extreme cases, this may produce a "fish-eye" or dead stone. If the pavilion angle exceeds the critical angle of a gem, the center will appear dark. A gemstone's critical angle is a function of its refractive index.
The term may refer to one of the following:
- The bottom quadrant or point on a stone with a pavilion that ends in a single point.
- A bottom facet cut parallel to the girdle on a stone with an otherwise pointed bottom, initially intended to prevent chipping.
Cutter or Polisher
Terms used in the diamond cutting industry to describe a person who cuts and polishes facets on diamonds. The person who performs the same functions on colored stones is known as a faceter.
A term used to describe the fashioning and/or faceting process for diamonds. For colored gemstones, this process is usually called faceting.
The depth of a cut stone can be measured from the table through the entire stone to the culet or keel. However, when ordering settings for a cut stone, the depth should be measured from the top of the girdle to the culet or keel.
A solvent used to dissolve dop wax. Faceters can also use isopropyl alcohol.
One of two classifications of gemstones. Diamonds come in all colors, including black and white, but regardless of color they are still classified as diamonds, not as colored stones.
A liquid solution containing finely ground diamond powder and a lubricating liquid (proprietary for each manufacturer), used for polishing surfaces or facets. Diamond slurries can be as coarse as 3,000 or as fine as 200,000 grit. You can purchase much coarser diamond grits, but I have yet to find them sold in a slurry version.
Dispersion of Light
When light passes through some minerals, it's split or dispersed into a spectrum of colors, like a prism. Diamonds famously disperse light into different colors, but some gemstones, like rutile and cerussite, can have far greater dispersion effects.
You attach gem rough to this metal post to secure it in a faceting machine quill for cutting or faceting.
A rack for storing your dops while not in use.
Most commonly, a shellac-based wax used to attach a stone to a metal post or dop.
The process of attaching a stone to a dop to prepare it for faceting, usually with dop wax. Other ways of dopping a stone include using super glue or epoxy.
When light passes through some minerals, the light beam splits into two separate beams. Gemologists can use this phenomenon to help identify certain gemstones without damaging them.
Eye Clear or Eye Clean
This refers to gems and gem rough with no evident flaws visible to the naked eye.
A flat surface on a stone or other media.
The process of cutting and polishing flat surfaces or facets on colored gemstones or other media. For diamonds, this process is usually called cutting.
This part of the faceting machine contains: the quill, the quill analog or digital angle guide, the primary and fine quill angle adjustments, the index wheel assembly, and the fine index adjustment.
A person who cuts and polishes facets on colored gemstones. People who do this on diamonds are usually called cutters.
The process of cutting and polishing facets on diamonds.
Faceters use flame sources, like oil lamps or butane torches, to heat up the dop or melt the dop wax when dopping or transferring a stone.
A cut and polished stone. From the Latin term gemma, meaning "engraved stone," referring to the first forms of currency as well as the relics of ancient societies found in ruins and burial grounds.
Plastic containers with foam inserts used for storing and viewing specimens in a gem collection. These jars may come with clear plastic or glass covers.
Developed for gemstone storage, these pre-folded papers are usually acid-free and can be wax or plastic coated.
The line created where the crown and pavilion facets meet, or the series of facets that separate the crown and pavilion facets.
Graspers or Gem Holders
A spring-loaded tool for picking up and holding a cut gem. Gem holders typically come in 3, 4, and 5 prong varieties.
Greenway Polishing Lap
A proprietary lap for polishing gems that contains chromium oxide polish.
Finely ground abrasive media for cutting or polishing stones. This media could be diamond, aluminum oxide, cerium oxide, chromium oxide, tin oxide, or other materials.
The edge of an incandescent bulb's reflection on a polished facet. Direct reflection can obscure defects on the surface of a polished facet. Only when the edge of the light reflection appears to pass the surface of the stone do some defects finally become visible.
When speaking of gemstones and minerals, this refers to scratch hardness and cutting resistance. Scratch hardness is measured on the Mohs scale from 1 to 10. Some minerals have cutting resistance, when a stone's crystal faces have different hardness values.
Also known as a positive stop, a solid mechanical contact on a faceting machine that prevents the quill from progressing beyond a preset point.
The part of a facet closest to the dop, sometimes referred to as the bottom of the facet.
A toothed gear on the quill of a faceting machine for setting the circumference position of a cut or facet. This gear allows the operator to cut or position numerous facets around the stone without changing the quill angle, thereby creating a tier of facets around the stone.
The marked number of a toothed position on an index gear. The positions are marked consecutively, from "0" to the highest numbered position on the specific index gear. The highest number also corresponds to position "0." Current standard gears may include 32, 33, 64, 72, 77, 80, 96, and 120 positions and will vary by manufacturer. However, index wheels of virtually any number, even single digits, can be produced on a custom basis.
To adjust all of the settings on a faceting machine to a nominal or zero value before beginning work on a new piece of rough.
The channel created in a stone as a saw blade cuts through it. Multiple factors determine the width of the kerf, such as the width of the blade, the thickness of the grit extending out from the sides of the blade, and any vibration the blade creates as it moves through the stone.
In faceting, a rotating disk impregnated with an abrasive for cutting or polishing stones. The cutting media is usually diamond grit or an oxide polish.
From the Latin word lapidarius meaning "of stone." A person who cuts and polishes stones is called a lapidary. The major lapidary arts include tumbling, cabbing, faceting, and carving.
A proprietary polishing lap that can be charged with diamond slurry.
A light source is necessary to clearly see the facets and adequately illuminate your workspace. Most faceting machines come with a light source included. You should consider the best light sources for different activities like cutting, grading, buying, and selling gemstones.
A proprietary polishing lap made of a hard resin. Lightning laps can come with different grades of polishing surfaces on each side, polishing grit on one side, a natural side without grit onto which a grade of diamond slurry can be applied, or as a topper. The coating on the laps is either cerium oxide or aluminum oxide. The natural sides can be coated with slurries of different coarseness. You can subsequently wash off the slurry and apply a finer slurry without fear of the coarser slurry being retained on the surface of the lap.
A magnifying device, usually hand-held, used for closeup viewing of gemstones.
Loupe-Visible or Loupe-Clean
Cut gems are usually evaluated with no more than a 10X loupe. If a flaw is noticeable at that magnification, it's referred to as loupe-visible. Loupe-clean means no flaws are noticeable under loupe magnification.
Whether on the crown or the pavilion, the main facets typically extend from the girdle to the table on the crown or from the girdle to the culet or keel on the pavilion. Some more complex designs may lack crowns, girdles, culets, or keels but still have main facets. However, in most cases, the main facets help reflect the most light back through the crown of the stone. They're usually cut at a stone's critical angle.
Marking Pen, Pencil, or Scribe
A felt marking pen, a graphite pencil, or an aluminum scribe come in handy for marking a piece of rough. A felt pen works well when preparing to cut a piece of rough from the main chunk, realigning a stone, or trying to keep track of which facet you're working on. A pencil or an aluminum scribe can make a discreet mark on a stone without damaging it or interfering with the dopping process.
On many faceting machines, the faceting head is attached to this fixture.
An especially flat and true lap-shaped disk with no grit on it. Used to support other laps such as toppers and Ultralaps, a master lap will often be used for truing or realigning a machine.
The junction of two facets.
The junction of three or more facets.
Methyl ethyl ketone, a solvent that will help dissolve or release epoxy from a stone or dop. It also dissolves dop wax much faster than alcohol.
A combination cut for a gemstone, which involves a brilliant-type cut above the girdle and a step cut below the girdle, or vice versa. Faceters often use a mixed cut to improve color and/or brilliancy or achieve a different visual effect.
A scale created to classify stones by relative resistance to scratching. The hardest substance on the Mohs scale is diamond, with a hardness of 10. The softest is talc, with a hardness of 1. However, the gradations of the Mohs scale aren't linear, only relative. In actuality, the absolute hardness of a diamond is 140,000, and that of talc is .03.
Gem material formed entirely by nature.
A hands free optical visor for magnified viewing.
The part of the stone below the girdle, otherwise the bottom portion of the stone.
On a design diagram, the view looking down at the crown through the vertical axis, up at the pavilion through the vertical axis, or at the silhouette of the stone 90° to the vertical axis.
Platen or Turntable
In a faceting machine, the rotating seat or platform on which a lap rests.
A unit of weight equal to 1/100 of a carat.
The smoothness of a facet's surface. A well-polished surface will be free of any visible unevenness, pitting, grooving, and scratches.
Polishing a Window
The process of cutting and polishing a large facet into one side of a piece of rough to look for flaws and color zoning inside the stone. This helps faceters plan how to get the most out of a faceted stone.
An unfinished stone that has the same general shape as the cutting design when viewed along the vertical axis.
Protractor or Analog Angle Indicator
The quill angle indicator on an analog faceting machine used to measure the angle of the quill relative to the lap.
Precious and Semi-Precious
These terms are no longer used in the jewelry industry to describe gems because they have too many exceptions to be useful. Currently, the preferred terms to describe gemstones are diamonds and colored stones. Diamonds occur in a full range of colors but are still classified as diamonds. All other stones, whether they have color or are colorless, are classified as colored stones.
The mechanism on the faceting head that holds the dop.
A clear liquid product that allows you to see through a crazed surface to the interior of a piece of rough of similar refractive index. With Refractol, you can examine the stone's interior in a non-destructive way. (Editor's note: Refractol may no longer be available, but you can use other commercially available refraction liquids as well as these common household liquids as substitutes).
The bending of light as it passes from one medium (air) through another medium (such as a solid).
Each transparent mineral slows the light that passes through it to a specific speed. The ratio of the speed of light in air to the speed of light through the mineral equals the mineral's refractive index or RI.
Also known as uncut stone, the material from which a faceter or cutter will produce a finished gemstone.
The degree to which light breaks up in a gemstone into multiple, smaller flashes of returning light.
The shape of the stone as viewed either up or down through the vertical access.
The view of a gem looking along the girdle plane, which allows a silhouette view of the crown and pavilion.
A single-cut gem has a single row of facets between the girdle and the table and a single row of facets on the pavilion.
A modification of a hard stop, which allows the quill to move past the set point. Faceters will note when they reach the set point by either an audible tone or a visible sign.
The ratio of the weight of a gemstone to the weight of an equivalent volume of water. You can learn to calculate this value. All minerals have particular specific gravity (SG) or density values, and gemologists can measure the SG of a gem to help identify it.
Splash Guard or Fence
A barrier around the lap platen wider than the laps, intended to catch water and swarf or debris that runs or spins off of a rotating lap.
The abbreviation for standard round brilliant cut.
A facet that has the same index number but a different quill angle than the facet above or below it, as in a step cut.
A dopping tool intended for initial dopping of rough. This tool has a shaft that fits into the transfer jig and a surface at one end with silicone putty. Faceters press the gem rough into the putty, which holds it during the dopping process.
The finely ground waste that comes off the stone during the cutting and polishing process.
This refers to the pattern of facets on a stone. There are two types of symmetry:
- Rotational, like a pie, in which each facet is the same as every other one.
- Mirror image, where all the lines and meetpoints on a diagram would be identical if folded down the Y-axis.
Symmetry may also refer to the alignment of facets on a diamond.
Gem materials made in a laboratory.
A facet on the crown, usually parallel to the girdle. In cases when the girdle isn't a straight line, the table is typically at 90° to the stone's center axis.
Table Adaptor or 45° Table Adaptor
A mechanism that attaches to the quill with the quill angle set at 45°. Faceters use it to hold the dop while cutting and polishing the table. It makes manipulating the stone against the lap easier.
A row of facets at the same elevation or quill angle around the stone, relative to the lap or cutting surface.
Thin metal laps with grit on one side. Usually much less expensive than standard laps, they need support from another lap or a master lap.
A device for transferring a stone from one dop to another accurately, so faceters can work on the opposite side of the stone.
A thin, very flexible plastic or Mylar-type disk with one side coated with a very fine polishing grade grit. An Ultralap is placed on top of a master lap or another lap and used to polish a stone. This lap style comes with a variety of grits, including diamond, aluminum oxide, tin oxide, and cerium oxide, and in a variety of grit grades.
Most faceting machines with protractors or analog scales that read in degrees have vernier scales. These allow faceters to further adjust the quill to quarters or tenths of a degree.
Also known as the rotational axis, an imaginary line extending through the center of the stone, from the center of the culet or keel to the center of the table.
Tom Mitchell is a jewelry designer in The Villages, Florida specializing in faceted colored gems, silver and gold jewelry, scrimshaw and cold connections designs. Tom teaches gem faceting at William Holland several weeks each year. mitchelljewelrystudio.com
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