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USFG Faceting DictionaryJan. 15, 1994, by Fred Van Sant 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. Alignment: The
positioning of the crown break facets across from the pavilion break facets,
as they are shown on the diagram sheet. Angle: Used
alone, ‘angle’ refers to the angle which a facet makes with the Girdle
Plane. 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. Apex: A single point
on the crown farthest from the Girdle Plane. 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. 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. Baguette: A small step-cut stone, usually
set in a row around a larger stone. 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. 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. 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. 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. 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. Brilliance: A general term used to describe
a stone’s overall appearance. It always includes Brightness,
and often includes Color Spread or Scintillation. C: The height of the crown as shown on the
diagram sheet. It is measured parallel to the vertical axis of the stone. 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. CAM Facets: The sets of facets--their angles
and indexes--which are used to make the CAM preform. Carat: A unit of weight. Equals 1/5 of a gram,
or 200 milligrams, or 100 points. CED: Center-to-Edge Distance.
This refers mainly to a girdle facet, and is the length of its normal intercept
on the girdle plane. Cheater: A bearing angle adjustment which
allows ‘indexing’ between teeth. This is usually a worm gear with a knob
that is turned by hand. Chip: A break-out of material from
the surface of the stone. Circumference Angle: See Bearing
Angle. 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. Color Spread: The
spreading out of white light rays into various colors after they enter
the crown and as they pass through the stone. Cone: A rolled facet shaped
like a cone, whose point is centered on the dop arm axis. Cone Point: The point of a cone, usually
made to serve as a temporary or permanent center point at the culet. 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. 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. Crown: That part of the
stone above the girdle. 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. 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. DPT: Degrees Per Tooth:
This is equal to 360 divided by the Gear Number. 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. 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. 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. 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. 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. Elevation Angle: The angle a plane or facet
makes with the girdle plane. See also Angle. Ellipse: A curving line whose every
point conforms to a precise mathematical formula. Elliptical: A stone shape whose perimeter
is an ellipse. 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. Facet Edge: See Line. Faceted Girdle: A girdle made up of a series
of flat facets. In the plan view they appear as a series of straight lines. Fixed Point: Any
point where four or more facets meet (as opposed to Floating
Point). 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. 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". Flaws: These are visible cleavage planes,fractures,
inclusions, gas bubbles, etc. If a flaw breaks the surface it becomes a
surface flaw. 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. Floating Point: A
point formed by only three facets (as opposed to Fixed
Point). 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. Freeform: See Baroque. Gear Number: The
number of teeth on the index gear; the highest numbered tooth on the gear. 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. Girdle Plane: A
theoretical plane inside the stone in the center of the girdle, at 90 degrees
to the vertical axis. Gram: The unit weight of the metric system.
One gram = 5 carats = 1000 milligrams. 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%. 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 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. Hardness: A material’s resistance to scratching
or abrasion. Hard Stop: Also called a "Positive Stop".
A solid mechanical metallic contact which prevents the dop arm from being
lowered past a set position. Index Gear: Toothed wheel used on faceting
machines for setting the circumference angle. Index Line: A line whose slope, divided by
DPT,
results in a whole number. 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. Index 0: Index Zero refers to the highest
number on any gear. This is useful for discussing gears in general, or
a general rule. 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. 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. Level Line: A line whose points are at the
same elevation. A line parallel to the Girdle Plane. Line: A line, or ridge,
or facet edge, on the stone is the juncture of two adjacent planes. Line Apex: A line
on the crown parallel to the Girdle Plane and
farthest from it. Line Culet: A line
on the Pavilion farthest from, and parallel
to, the Girdle Plane. 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. 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. Man-Made Material: A faceting material made
by man which has no corresponding content and structure in nature. See
Synthetic
Material for contrast. 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. 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. Meet: A term used in judging how well facets
come to a point. 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. 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. Natural Material: A faceting material wholly
formed by nature. 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. Normal Intercept:
Same as a Normal except the distant point is the
Point
of Origin. OM, Offset Multiplier: The number multiplied
by the stone width W to obtain the amount of offset for rolled girdle arcs. 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. Oval: A shape which is more or less elliptical. P: The height of the pavilion as shown on
the diagram sheet. Pavilion: That part
of the stone below the girdle. Pendeloque: A pointed pear shape. Plane: A geometrical flat surface, having
infinite two-dimensional extension. Flat facets are restricted areas of
the plane on which they lie. Plan View: A view looking down the stone’s
Vertical
Axis at the Crown, or up the Vertical
Axis at the Pavilion. 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. Point: A unit of weight. Equals 1/100 of a
carat. 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. 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. Protractor: The angle indicator on a faceting
machine. The protractor should read 90°
when the dop arm is parallel to the lap. Quill: Part of the faceting machine that holds
the dop, also called the dop arm. Race-Track Oval: An oblong shape whose sides
are straight parallel lines and whose ends are half a circle. Reference Point: Any point on the stone, permanent
or temporary, to which a facet can be cut (a catch-all term). Refraction: In general, the bending of a light
ray as it passes from one medium into another. 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. 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). RI: See Refractive Index. 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". Rose Cut: A faceted
stone having a crown with no table and without a pavilion. Scratch: A linear gouging on the facet’s surface. Scintillation:
The degree of break-up of light passing through a faceted stone into more
and smaller flashes of returning light. 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. 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. Shape: The stone’s shape is its outline form
when viewed along the Vertical Axis. 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. 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. Slope: The angle a line makes with respect
to the X-Axis, as measured on the Girdle
Plane. Sloping Table: A large facet on the crown
which functions like a table, but is not parallel to the Girdle
Plane. 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. SRB: Standard Round Brilliant. Staggered Facets: A set of facets whose index
numbers evenly split the index numbers of the set of facets below it or
above it. Step Facet: A facet having the same Index
Number as an Adjacent Facet. 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. Rotational symmetry is illustrated by a pie, where
each slice is the same as every other slice. 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. 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. 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. T: Table width as shown on the diagram
sheet. Table: A facet on the
crown parallel to the Girdle Plane. 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. 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. 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). Uniformity: The degree
of sameness of the facets within a facet set as to their size and shape
(or mirror-image shape). 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. 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. View Arrow: An arrow on the diagram sheet
which indicates, in a sideways view, how the crown plan view is being viewed. W: The width of the stone as shown on a diagram
sheet. Usually at 90°
to the Length dimension. 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. 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. Z-Axis: An imaginary line at right angles
to the X and Y axes. Usually the same as the vertical axis. 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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