USFG Faceting Dictionary

Jan. 15, 1994, by Fred Van Sant
Revised Sept. 15, 1999

(Note: For terms dealing with crystal structure, geology, and other such technical subjects, see a textbook.)

Adjacent Facets:Two facets on a stone which form a line between them. If facet A shares a line with facet B they are adjacent.
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Alignment: The positioning of the crown break facets across from the pavilion break facets, as they are shown on the diagram sheet.
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Angle: Used alone, ‘angle’ refers to the angle which a facet makes with the Girdle Plane.
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Angle of Incidence: A ray striking a facet’s surface at 90° has an angle of incidence of zero; its deviation from 90° is its angle of incidence.
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Apex: A single point on the crown farthest from the Girdle Plane.
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Apex Facets: All facets meeting at the Apex. If there is a Line Apex, then these are both facets forming the line plus all facets meeting at either of the two points on the end of the line.
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Arc: A smoothly curving line which is a segment of a circle. The term “arc” usually refers to a girdle section made using an offset device.
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Baguette: A small step-cut stone, usually set in a row around a larger stone.
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Barion Cut: A type of cut that retains the brilliant type culet area by having steep half-moon facets hanging down from the girdle. The crown is often a step-cut.
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Baroque, or Freeform: A stone which is non-symmetrical in shape, and lacks repetition of a pattern of facets at regular intervals. It has neither rotational nor mirror-image symmetry.
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Bearing, Bearing Angle: The angle of a facet’s normal intercept, as measured on the girdle plane. This is also called the “Circumference Angle”, and is used mainly for describing girdle facets. The Zero Degrees line lies on the X axis, to the right of the Point of Origin. On faceting machines, the bearing angle is the tooth of an index gear times DPT.
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Breakpoint Method: A special sequence of meetpoint cutting which makes the shape. A set of facets is cut to a center point. Then a set of Break Facets is cut in a chaining manner. Then a set of 90° girdle facets is cut using the Break Facets’ index numbers, to make a level girdle line. This produces the shape.
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Brightness: The amount of light returned back through the crown, expressed as a percentage of the light which entered the crown. The determination of brightness may be expressed as a subjective evaluation, or as the result of a computer analysis; in the latter case it is relative to the lighting model used and other program characteristics.
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Brilliance: A general term used to describe a stone’s overall appearance. It always includes Brightness, and often includes Color Spread or Scintillation.
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C: The height of the crown as shown on the diagram sheet. It is measured parallel to the vertical axis of the stone.
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CAM: The Centerpoint Angle Method of making a preform. 1) Facets are cut to a temporary centered culet point at given angles and indexes. 2) The dop arm is then set parallel to the lap (90° on protractor), and facets are cut using the same index numbers to make a level line around the stone. This results in the desired preform shape.
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CAM Facets: The sets of facets–their angles and indexes–which are used to make the CAM preform.
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Carat: A unit of weight. Equals 1/5 of a gram, or 200 milligrams, or 100 points.
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CED: Center-to-Edge Distance. This refers mainly to a girdle facet, and is the length of its normal intercept on the girdle plane.
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Cheater: A bearing angle adjustment which allows ‘indexing’ between teeth. This is usually a worm gear with a knob that is turned by hand.
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Chip: A break-out of material from the surface of the stone.
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Circumference Angle: See Bearing Angle.
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CLAM: The Corner Locator Angle Method for locating a corner on a preform. For the pavilion, two temporary facets make a line from culet to corner.
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Color Spread: The spreading out of white light rays into various colors after they enter the crown and as they pass through the stone.
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Cone: A rolled facet shaped like a cone, whose point is centered on the dop arm axis.
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Cone Point: The point of a cone, usually made to serve as a temporary or permanent center point at the culet.
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Continuous Girdle: One which has no vertical break line, as in an oval or circular stone where the entire girdle is polished by rolling the stone on the lap.
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Critical Angle: The angle of incidence at which total reflection occurs. At the surface of two differing mediums, usually the gem material and air, if the light ray is below the critical angle of incidence, light will pass from one medium to the other. See Angle of Incidence.
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Crown: That part of the stone above the girdle.
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Culet: 1) A small facet parallel to the girdle plane made to prevent chipping the culet point; 2) A single point on the pavilion farthest from the Girdle Plane; 3) A bottom line on the pavilion parallel to the Girdle Plane (also called culet line); 4) Definition 2 or 3 plus the facets attached – the area of the culet.
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Culet Facets: All facets meeting at the culet. If there is a Line Culet, then these are both facets forming the line plus all facets meeting at either of the points on the end of the line.
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DPT: Degrees Per Tooth: This is equal to 360 divided by the Gear Number.
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Direction: The rotational direction in which the index numbers increase in magnitude on the diagram sheet. It is either Clockwise (CW) or Counterclockwise (CCW). The highest number on the index gear normally lies on the Y axis of the plan view.
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Dispersion: Gemology–See dispersion charts for various minerals. Generally–the degree to which white light passing through the stone is changed into various colors; “Color Spread” is preferable for describing finished cut stone performance.
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Double Rose Cut: Same as Rose Cut but having a pavilion which is shallow like the crown and usually is a duplicate of the crown pattern.
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ECED: (Equal Center to Edge Distance) This term refers to the length of an imaginary line on the girdle plane which extends from the center of the girdle plane to connect at 90 degrees with a girdle facet, as seen in a plan view. A design labeled ECED indicates the stone can be preformed at a single mast height setting. See also Normal Intercept.
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Elevation: The height of a point above the girdle plane. In discussing pavilion points, assume stone is held upside down as you would normally do when examining the pavilion, so all point elevations are positive.
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Elevation Angle: The angle a plane or facet makes with the girdle plane. See also Angle.
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Ellipse: A curving line whose every point conforms to a precise mathematical formula.
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Elliptical: A stone shape whose perimeter is an ellipse.
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Facet: Any bounded surface on the stone. A facet is not a plane, but flat facets are small areas of the plane on which they lie.
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Facet Edge: See Line.
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Faceted Girdle: A girdle made up of a series of flat facets. In the plan view they appear as a series of straight lines.
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Fixed Point: Any point where four or more facets meet (as opposed to Floating Point).
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Fisheye: If the stone is held closely over newsprint which can be read by looking straight down through the table and through the main culet facets, it is a fisheye.
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Flatness: If a facet is perfectly flat, the entire surface of the facet lies on a single geometrical plane. If any part of the facet curves away from the exact plane, it is called “rounding”. If a break line is seen on the facets surface it is not flat–it is called a “double-facet”.
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Flaws: These are visible cleavage planes,fractures, inclusions, gas bubbles, etc. If a flaw breaks the surface it becomes a surface flaw.
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Floating Facet: A facet whose perimeter points are all floating points. The facet is not anchored to a fixed point, so its size is adjustable.
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Floating Point: A point formed by only three facets (as opposed to Fixed Point).
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Four-Part Design: A stone whose facet pattern consists of four quarters which mirror each other BOTH left/right AND top/bottom in the plan view, such as the Oval, Square, Rectangle, Lozenge, and Marquise. It has a symmetry of 2Y or 4N.
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Freeform: See Baroque.
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Gear Number: The number of teeth on the index gear; the highest numbered tooth on the gear.
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Girdle: The girdle is a thin cross-section of the stone situated between the Crown and the Pavilion and at 90 degrees to the vertical axis. Its perimeter may be faceted or continuous or a combination of both, and is usually parallel to the vertical axis.
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Girdle Plane: A theoretical plane inside the stone in the center of the girdle, at 90 degrees to the vertical axis.
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Gram: The unit weight of the metric system. One gram = 5 carats = 1000 milligrams.
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GVF: The girdle volume factor is a mathematical constant used to determine the volume of the Girdle. Girdle Volume = GVF x W x W x W, based on the girdle thickness being 2% of W. The Volume Factor (VF) includes the 2% girdle; GVF is used only to adjust the volume if the girdle varies from 2%.
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H: H is the total height of the stone as shown on the diagram sheet. It is expressed as a decimal fraction of W. H/W=C/W + P/W + .02
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Half-Light: The greyed edge of an incandescent bulb’s reflection on a polished facet. For observing the surface of a polished facet, the direct reflection of the light blinds the eye and does not reveal errors on a facet’s surface as well as half-light does.
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Hardness: A material’s resistance to scratching or abrasion.
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Hard Stop: Also called a “Positive Stop”. A solid mechanical metallic contact which prevents the dop arm from being lowered past a set position.
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Index Gear: Toothed wheel used on faceting machines for setting the circumference angle.
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Index Line: A line whose slope, divided by DPT, results in a whole number.
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Index Number: The number of the tooth on the index gear. Teeth are numbered consecutively in whole numbers, starting from the highest number, which is also number zero, around the gear.
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Index 0: Index Zero refers to the highest number on any gear. This is useful for discussing gears in general, or a general rule.
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Keel: The bottommost line on the pavilion, one end of which is the culet point. This is like a Line Culet except that it is not parallel to the Girdle Plane.
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L: The length as shown on a diagram sheet. This is normally the longest dimension on the Girdle Plane. It usually lies along the X or Y axis.
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Level Line: A line whose points are at the same elevation. A line parallel to the Girdle Plane.
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Line: A line, or ridge, or facet edge, on the stone is the juncture of two adjacent planes.
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Line Apex: A line on the crown parallel to the Girdle Plane and farthest from it.
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Line Culet: A line on the Pavilion farthest from, and parallel to, the Girdle Plane.
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L/W: The length/width ratio, where length and width are designated on the diagram. Both measures are on the Girdle Plane. L/W is never less than 1.
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Main Facets, Crown or Pavilion: A set of large facets which extend from Girdle to Table on the Crown, or from Girdle to Culet on the Pavilion.
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Man-Made Material: A faceting material made by man which has no corresponding content and structure in nature. See Synthetic Material for contrast.
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Max. CAM Angle: This is the highest angle usable when making a CAM preform on an ECED shape, which will contain all the pavilion facets; it allows the culet point made to be permanent.
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Max. Cone Angle: This is the highest angle usable for a cone culet facet (rolled) which will contain the given single CAM angle or set of CAM preform angles.
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Meet: A term used in judging how well facets come to a point.
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Meetpoint: Any point on the stone used in meetpoint cutting. A point to which a facet is cut to meet. This is a functional term.
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Mohs Scale: The most commonly used scale for approximating the relative hardness of minerals: 1-talc; 2-gypsum; 3-calcite; 4-fluorite; 5-apatite; 6-orthoclase; 7-quartz; 8-topaz; 9-corundum; 10-diamond.
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Natural Material: A faceting material wholly formed by nature.
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Normal: The bearing and distance of an imaginary straight line in 3D space, which is connected to a line or plane at 90 degrees, and connects at its other end to a distant point. The shortest distance between a line or plane and a distant point.
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Normal Intercept: Same as a Normal except the distant point is the Point of Origin.
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OM, Offset Multiplier: The number multiplied by the stone width W to obtain the amount of offset for rolled girdle arcs.
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Omni Preform: A special sequence of Meetpoint cutting which makes the shape. A set of facets is cut to a center point. Then a set of 90° facets is cut, using index numbers different from the culets, in a chaining fashion. The shape is made but the girdle is not level.
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Oval: A shape which is more or less elliptical.
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P: The height of the pavilion as shown on the diagram sheet.
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Pavilion: That part of the stone below the girdle.
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Pendeloque: A pointed pear shape.
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Plane: A geometrical flat surface, having infinite two-dimensional extension. Flat facets are restricted areas of the plane on which they lie.
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Plan View: A view looking down the stone’s Vertical Axis at the Crown, or up the Vertical Axis at the Pavilion.
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Point of Origin: The center of the Cartesian, or Rectangular, coordinate system, where the X axis, the Y axis, and the Z axis intersect. The center of the girdle plane.
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Point: A unit of weight. Equals 1/100 of a carat.
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Polish: The smoothness of the surface of a facet. A well-polished facet is free of any unevenness, pitting or grooving, and of all traces of the cutting done by the previous lap.
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Preform: A piece of rough material with a girdle at 90° which, when the stone is viewed along the vertical axis, has the same shape as the design to be cut.
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Protractor: The angle indicator on a faceting machine. The protractor should read 90° when the dop arm is parallel to the lap.
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Quill: Part of the faceting machine that holds the dop, also called the dop arm.
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Race-Track Oval: An oblong shape whose sides are straight parallel lines and whose ends are half a circle.
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Reference Point: Any point on the stone, permanent or temporary, to which a facet can be cut (a catch-all term).
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Refraction: In general, the bending of a light ray as it passes from one medium into another.
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Refractive Index: Index of Refraction, represented by a number or pair of numbers. See a book on Gemology, as this is too complex to go into here.
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Rhomboid: A stone whose general shape is that of a parallelogram. It has no mirror-image symmetry and its rotational symmetry is 2 (Sym = 2N).
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RI: See Refractive Index.
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Rolled Girdle: A girdle section made by setting the dop arm in free-wheeling, and turning it while grinding or polishing the girdle. If there are no vertical break lines then is called a “continuous girdle”.
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Rose Cut: A faceted stone having a crown with no table and without a pavilion.
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Scratch: A linear gouging on the facet’s surface.
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Scintillation: The degree of break-up of light passing through a faceted stone into more and smaller flashes of returning light.
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Semi-fisheye: Same as fisheye, except that newsprint may be read only by very slightly tipping the stone off from the line of sight along the Vertical Axis of the stone.
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Set, or Sets, of Facets. Facet Set: 1) Several facets having the same shape or mirrored shape, the same surface area and angle, which occur at patterned intervals around the stone.  2) Whatever occurs on the same line in a facet list on a diagram sheet.
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Shape: The stone’s shape is its outline form when viewed along the Vertical Axis.
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Side View: The side view is any view looking along the Girdle Plane, at right angles to the Vertical Axis, with both Crown and Pavilion in equal view. The terms Top View, Bottom View, Left View, and Right View are all side views, which refer to the edge of the diagram sheet from which the crown plan view is being observed.
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Single Cut: A stone whose crown consists of a single row of facets between girdle and table, and a single row of facets on the pavilion.
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Slope: The angle a line makes with respect to the X-Axis, as measured on the Girdle Plane.
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Sloping Table: A large facet on the crown which functions like a table, but is not parallel to the Girdle Plane.
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Soft Stop: A modification of a hard stop which allows the dop arm to move further downward under pressure after contact is made. The amount of further arm movement is metered by a micrometer gauge or by an electrical measure.
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SRB: Standard Round Brilliant.
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Staggered Facets: A set of facets whose index numbers evenly split the index numbers of the set of facets below it or above it.
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Step Facet: A facet having the same Index Number as an Adjacent Facet.
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Symmetry: This refers to the patternof facets on the stone. In general parlance, there are two kinds of symmetry, rotational symmetry and mirror-image symmetry.
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Rotational symmetry is illustrated by a pie, where each slice is the same as every other slice.
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Mirror-image symmetry is illustrated by folding a design plan view along its Y axis so that the two halves overlap, with points and lines coinciding.
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A design may have both mirror-image and rotational symmetry. The term “symmetry” should not be used to denote similarity of shape among facets of a set; that is referred to as “Uniformity“. Also the term Symmetry should not be used to describe the outline of the stone or the outline of the table as to how even it is.
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Synthetic Material: A faceting material which is made by man to imitate a natural material. It has a content and structure that corresponds to a natural material.
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T:  Table width as shown on the diagram sheet.
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Table: A facet on the crown parallel to the Girdle Plane.
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Tangent Ratio: The ratio of the tangent of one angle to the tangent of a second angle. Used for translating one set of angles to another while holding the plan view constant.
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Tier: A tier of facets consists of a group of facets at the same elevation around the stone. It may be made up of one or more sets of facets.
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Two-Part Design: A stone whose facet pattern consists of two halves which mirror each other left/right in the plan view, such as the Pear and Heart. It has a Symmetry of 1 and mirror-image (Sym =1Y).
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Uniformity: The degree of sameness of the facets within a facet set as to their size and shape (or mirror-image shape).
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Vertical Axis: An imaginary straight line through the stone which, as in a standard round brilliant, runs through the center of the table and through the culet point.
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VF: Volume Factor. A mathematical constant, unique for each design, for determining a stone’s volume from its width W. Volume=VF x W x W x W.
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View Arrow: An arrow on the diagram sheet which indicates, in a sideways view, how the crown plan view is being viewed.
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W: The width of the stone as shown on a diagram sheet. Usually at 90° to the Length dimension.
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X-Axis: An imaginary line passing left-to-right through the center of the plan view. The left-right direction on the diagram sheet is when it is held in the position to read the text.
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Y-Axis: An imaginary line passing top-to-bottom through the center of the plan view. The vertical direction on the diagram sheet is when it is held in the position to read the text.
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Z-Axis: An imaginary line at right angles to the X and Y axes. Usually the same as the vertical axis.
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Zero Vertical Loss Method: A method used to compute CAM angles, such that the culet point made can be the permanent culet point.
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