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GIA Scales GIA Diamond D-Z Color Scale
DIAMOND CUT
The cut of a diamond determines how it reflects light, which is responsible for its sparkle or brilliance. A well-cut diamond is cut by a skilled professional to the best proportions possible so that light will be reflected from each of its
mirror-like facets and disperse through its top. Too shallow a cut will let light escape through a diamond's bottom, causing it to appear dull, while too deep a cut will allow light to be lost through a diamond's sides, making it appear dark.
Because a diamond with perfect color and clarity could nevertheless have poor brilliance if it is not well cut, many gemologists consider this to be the most important property to note when choosing a diamond. Diamonds can carry cut grades of Excellent, Ideal, Very Good, Good, or Fair.
The traditional round cut provides a combination of the best qualities a diamond can have. However, there are other shapes that can emphasize a diamond's different features. The choice of a shape is best based on personal taste and the design of the jewelry.
In 1919, a European cutter and mathematician
presented his Doctoral Thesis depicting the "Ideal Proportions"
of a round diamond which result in maximum brilliance and fire. When
achieving these, or close to these proportions, the more lively, brighter
and visible spectral colors appear from the crown facets.
In addition to the angles and proportions, the
finish, which includes the overall polish of the diamond, the condition of
the girdle (thickness and texture) and culet (not being nicked, chipped,
abraided, out of alignment or off center), and the symmetry of the facets
(meeting at the apexes and correctly positioned) are essential to the
beauty and appearance of the well cut and proportioned diamond.
Diamonds are cut from the rough, natural state. The
diamond rough is studied thoroughly and diligently by an expert called the
planner. The planner's responsibility is to decide how the diamond rough
should be cut to yield the best value. To insure the greatest value, the
planner must have the skills and instincts of a cutter/craftsman as well
as good business acumen. After marking the diamond with India ink the more
physical tasks of sawing and/or cleaving, girdling, faceting and polishing
are peformed.
Remember, the cut of a diamond is not the shape of
the diamond. The most common shapes seen are round, oval, marquise, pear,
emerald cut and heart shape. The most brilliant of these is the round
brilliant. The following are some average cutting times for specific
diamond shapes:
- Round brilliant............2-4 hours
- Oval brilliant...............8-10 hours
- Marquise shape..........12 hours
- Emerald cut................14-16 hours
GRADING THE CUT
Grading a diamond's cut and proportions are more
complicated than grading clarity and color. The grading can be done by
sight, measurements, or by the more accurate proportion scope. Four
measurements are taken for
accurate, proper grading. These are:
- Table diameter
- Crown height
- Girdle thickness
- Pavilion depth
TABLE
This measurement is taken with a technical
instrument such as the Leverige gauge; but, more efficiently with a table
gauge. The table size is measured from corner to corner. The measurement
is then converted to a percentage by dividing the longest table
measurement by the average girdle diameter and mutiplying by 100%.
Hint: You can generally determine if a round diamond
is well proportioned by examining a combination of star and table facet
sides. Sides that bow out or that are straight (in red) are indicative of
poorer cut stones. Table sides bowed in slightly (in blue) are of a well
cut stone.
CROWN
Crown height is computed using the angle of the
crown and the size of the table . Crown angles steeper than the ideal 34.5
degrees (in red) usually have large tables and thick crowns; crown angles
less than the ideal 34.5 degrees (in green) usually have small tables and
shallow crowns. The brilliance, scintillation and spectral/prismatic
colors (dispersion) are decreased the further they are from the ideal.
Important: If the table size is large and the crown
angle is less than 34.5 degrees (in yellow) you have what is called a
fisheye, a very spready stone that appears larger from the top than its
weight would suggest but has sacrificed beauty.
GIRDLE
Girdle size needs to be ideal (.7-1.7%) or near
ideal, resulting in thickness necessary for setting security and safety
from chipping but not too thin or too thick. Thick girdles indicate a
stone that is carrying an unnecessary amount of weight in the center.
Girdles can be textured (frosted or sugar cube appearance), polished or
polished and faceted. Apexes should meet crown and pavilion facet
junctions all the way around. It should not have a wavy appearance as it
circles the stone. It should be symmetrical and straight. The thickness
should be uniform.
PAVILION
Pavilion depth may be judged by looking through the
table (viewing the diamond from the top/crown). The best cut is indicated
when the table reflection is approximately 1/3 of the table size,
indicating a pavilion angle close to 40.75 degrees. The depth of the
pavilion will be about 43.1% of the girdle diameter in this case. Look
down into the stone to see the table reflection. Light travels through the
table facet and down into the stone. It gets reflected back up by the
facets in the pavilion of the stone. It is like looking at a mirror image
of the table reflected off the bottom of the stone and may have small dark
bowtie shapes surrounding the grey reflection of the table image.
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Jewelry at Auction - Glossary of terms
