Fluorescence
Some gem-quality diamonds fluoresce: they emit
light when exposed to long-wave ultraviolet light. How does this fact affect a diamond's
appearance and value? In the past, some people in the diamond trade have considered
moderate to strong fluorescence as a negative value factor for fine diamonds and a
positive value factor for diamonds with a lower body color. Why? The trade perceives
diamonds without fluorescence as "more pure" than diamonds with it. There is a
perceived rarity for diamonds of fine color without fluorescence. Fine quality diamonds
with strong fluorescence may be undervalued because rare extremely strongly fluorescent
diamonds known as "overblues" have a visible haziness that makes them appear
almost cloudy in light with strong ultraviolet content. Dealers have theorized that strong
fluorescence may affect apparent clarity. At the same time, strongly fluorescent diamonds
with a yellowish body color have long been considered to appear to have a better color
because the blue of the fluorescence makes them appear more white in sunlight, which is a
source of ultraviolet light. Lower-color diamonds with strong fluorescence sometimes
command a premium. The Gemological Institute of America's Gem Trade Laboratory, the
country's leading diamond grading lab, lists fluorescence as an identifying
characteristic, not a grading factor. And based on a GIA GTL random sample of data for
26,000 diamonds, diamonds with fluorescence are more rare than nonfluorescent stones: 65
percent of diamonds have no reported fluorescence. Listed below is GIA's list of
abbreviations for strength of fluorescence as well as their meanings:
- N No
Fluorescence
- F or FB Faint
or Faint Blue Fluorescence
- SL Slight
Fluorescence
- M Medium
Fluorescence
- S Strong
Fluorescence
- EF Extreme
Fluorescence
To help clear up some of the confusion around fluorescence, GIA commissioned a study on
the impact of fluorescence on the appearance of diamonds and published the results in its
professional gemology journal, Gems & Gemology in the Winter 1997 issue. The
fluorescence study compared sets of round brilliant diamonds with a range of color grades
in different controlled lighting conditions. The diamonds were evaluated by gemologists,
dealers, and untrained observers. The results? Untrained observers could not distinguish
any affects of fluorescence. Even trained observers did not consistently agree. In
general, strongly blue fluorescent diamonds were judged to have a better color grade in
the face up position. (No affect of fluorescence was observed in the table-down position,
which is how diamonds are color graded by laboratories.) No relationship between
fluorescence and transparency was apparent. The effect of fluorescence on color was most
notable in grades I through K. The conclusion? "In the table up position (as is
commonly encountered in jewelry), diamonds described as strongly or very strongly
fluorescent were, on average, reported as having a better color appearance than less
fluorescent stones. In this study, blue fluorescence was found to have even less effect on
transparency. These observations confirm GIA GTL's experience grading millions of diamonds
over the decades." The study seems to indicate that over the years, the trade has
unfairly stigmatized diamonds with strong fluorescence. And these diamonds are often
available at a discount because of trade perceptions of possible negative impact of
fluorescence on the salability of stones. Because "overblues" are so rare, none
of these diamonds were included in the GIA study. Overblues clearly do have a different
appearance than ordinary strongly fluorescent diamonds. If you are ever in the Smithsonian
institution in Washington D.C., you can see the most famous example of an overblue: the
127 carat Portuguese Diamond. While you are there, make sure to visit the famous Hope
Diamond, which owes some of its legendary curse to the fact that it fluoresces an
extremely unusual red, which is only known to happen in blue diamonds.
Girdle
The girdle is the outer edge of a diamond. The
grade of a diamond's girdle is determined by the appearance of the girdle at its thinnest
point and thickest point. A diamond's girdle can be faceted, polished smooth, or have a
slightly granular appearance. Very fine cut diamonds often have faceted girdles. A diamond
cutter must spend extra time to carefully facet a girdle's edge. A faceted girdle does not
improve a diamonds grade. Most labs grade only the thickness of a diamonds girdle and not
the surface appearance.
Ideal girdle thickness should range between Very Thin to
Thick. Sometimes a diamond can have a perfect medium girdle around ninety-nine percent of
its diameter and only be very thick at one very minute, isolated point. This diamond will
receive a laboratory girdle grade as medium to very thick. Diamonds that have grades
extremely thin, very thick or extremely thick are usually not recommended.
The Length-to-Width Ratios
Length-to-width ratios aren't the kind of thing most people think about when
purchasing a diamond. However, this is an important factor that shouldn't be overlooked
when buying diamonds of certain shapes. The length- to-width ratio is based on the
proportions of the original rough crystal. Usually, the cutter is reluctant to sacrifice
weight and value to meet the established ideal proportions for the chosen shape.
Length-to-width ratio is only important in fancy cuts or diamonds that are not round. The
length-to-width ratio, quite logically, is calculated by dividing the length by the width.
Polish
Polish influences how well light is able to pass
through a diamond and is very important to a diamond's brilliance. It is therefore
necessary to select a diamond which has a polish that is laboratory certified to be Good,
Very Good or Excellent. Diamonds that have poor to extremely poor polish are less
brilliant because they have microscopic polish lines that blur the surface of the diamond.
These polish lines reduce the amount of light that enters or exits a diamond. When
selecting your diamond, be aware many diamonds possess poor polish. Diamond cutters can
greatly reduce labor costs by not taking time to properly polish a diamond.
The following are grades used to designate the quality
of a gem's polish:
- EX or E--Extremely Good
- VG or VGD--Very Good Very
difficult to locate under 10 X power
- GD or G--Good Difficult
to see under 10 X Power
- FR or F--Fair Easy to see under
10 X Power
- PR or P--Poor Very easy to see
under 10 X / difficult to unaided eye
- VP--Very Poor Relatively easy to
see with the unaided eye
- EX or EP--Extremely Poor Obvious
to see with unaided eye
Symmetry
Symmetry is a crucial element of a quality finished diamond. By symmetry here we mean
the exactness of the shape and the balanced arrangement of the facets. To the unaided eye,
finish features usually have little effect on appearance. Symmetry is less important in
diamonds that have lower clarity grades. When selecting your diamond, make sure the
grading report rates it as Excellent, Very Good or Good.
Table
The table is the large flat facet on the top of a diamond. The table has a direct effect
on the sparkle of a diamond. The size of the table in concert with the angle of the crown
is responsible for the balance between brilliance--the flashes of white light bouncing
back to the eye from within the diamond--and the play of colors created by refracted light
as it prisms its way through a diamond's facets. The following are the ideal table
percentages for various shapes of diamond:
- Emerald 50%
- 75%
- Marquise 50%
- 62%
- Oval
50% - 62%
- Pear
50% - 62%
- Princess
65% - 80%
- Radiant 65%
- 80%
- Round 52.4%
- 57.5%