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CUT
Cut/make: How well it is cut determines a diamond's brilliance. It is the skill of the cutter that unlocks the natural beauty of a diamond, revealing all of its hidden fire and brilliance. Most diamonds are cut with a full 58 facets and it is important that these are cut to precise angles. A diamond that is cut well will reflect light from one facet to another and then back out through the top of the diamond. Only a well-cut diamond will really sparkle.
Definition

In its rough state, a diamond's beauty is well concealed.  Through cutting, the magnificent optical beauty of a diamond is revealed.  A diamond cutter's challenge is to balance beauty with weight retention from the rough diamond crystal.

In the best-case scenario, approximately 50% of the weight is lost from the original crystal in the cutting process.  Because size is important to many consumers, often cutters sacrifice diamond beauty in order to save weight, maximizing the size of the finished diamond.  Note that cut is also referred to as make.

Shapes:

When the term "cut" is mentioned, most consumers think of shape, but shape and cut are different.  Diamonds are cut and fashioned in different shapes, including round brilliant, pear, oval, marquise, princess cut, emerald cut (rectangular), heart, and triangle.  The most popular shape is the round brilliant.  The other shapes are referred to as fancy-shapes.  Round brilliant diamonds retain less weight from the rough diamond crystal, and are generally more valuable than fancy-shaped diamonds.

Optical Beauty:

The way a diamond reflects and refracts light is dazzling to the beholder.  There are four factors that determine the optical beauty of a diamond: luster, brilliance, dispersion, and scintillation.

Luster	The quantity and quality of light reflecting from the surfaces of a diamond.
Brilliance	The amount of white light returned to the eye from the diamond.
Dispersion	The amount of rainbow colors returned to the eye from within the diamond.
Scintillation	The sparkle, which is the combination luster, brilliance, and dispersion when there is movement by the wearer or light source.

How Cut is Graded

In the past, diamonds were analyzed, not graded, for cut by visual estimation.  Today, there are several electronic machines used to determine the angles and proportions relating to the quality of cut.  A diamond's finish, including polish and symmetry, is graded by human examination through a binocular microscope.

Round Diamonds - The standards for quality of cutting a round brilliant diamond have been established by AGS and GIA using nearly 100 years of research.  AGS and GIA Laboratories assign a numeric cut grade, complete with all the proportions and finish parameters for round diamonds.

Fancy Diamonds - Since the proportions for fancy-shaped diamonds vary the optimal balance of luster, brilliance, dispersion and scintillation, is different for each shape.  Therefore, no cut grade is assigned for fancy-shapes at this time. 

Terminology

Ideal  Cut	A diamond cut to optimal proportions, with optimal polish and symmetry, with the most weight loss to produce maximum luster, brilliance, dispersion, and scintillation.  Diamonds cut to this standard are the most valuable, with only 5% of the round brilliant diamonds on the market cut to this standard.
Well- Cut	Diamonds that have very good optical beauty that fall just outside of the parameters of Ideal Cut diamond.  These diamonds are priced less than Ideal cuts because they are not as rare.
Deep Cut	This diamond will appear smaller than it weighs because it's weight is retained in the depth.  It is cut with a deep pavilion (bottom of the diamond) that does not reflect light back through the crown (top of the diamond).  Light leaks out the pavilion producing a dark appearing diamond that lacks beauty.  These diamonds are sometimes called nailheads due to their dark, face-up appearance.
Shallow Cut	Diamonds that are cut with shallow pavilions that do not reflect light back through their crown.  The light leaks out from the pavilion, producing a washed-out or watery appearance that is not beautiful.  Weight is retained in the diameter, making the diamonds appear larger than they weigh.  These diamonds are sometimes called fisheyes, due to unsightly reflections in the crown area.
Bow- Tie	Effect dark area in the center of some fancy-shaped diamonds.  A large bow-tie shape in the center of a fancy shaped diamond detracts from beauty and lowers the value.

Cut in Relation to Value

Cut is the most important factor to a diamond's beauty.  Regardless of the color, clarity, and carat weight, a well-cut diamond will be beautiful.  Cut is so important to the value of a diamond that it can affect the value from 25% to over 50%. 

Fancy-shaped diamonds, since they retain weight from unusual shaped rough crystals, are often less expensive then comparable round diamonds.  Carat for carat, since fancy-shaped diamonds are elongated, they appear larger than round diamonds.  If a consumer is interested in maximizing size appearance, they can buy a smaller, yet larger-appearing fancy-shaped diamond of the same quality for less than a comparable round brilliant diamond.

COLOR
Colour: Most gem diamonds seen alone appear white, but most have a hint of colour, mainly yellow. Diamonds with no trace of colour at all are extremely rare and the closer they are to no colour, the more rare and valuable they will be. Also rare are diamonds with a strong pure colour that are called "fancies" and which are very valuable. The Argyle Mine in Australia, which is the world's biggest diamond mine, produces the rare pink diamonds as well as beautiful cognac and champagne coloured diamonds.
Definition

The amount or presence of body color in a diamond.

Diamonds come in all colors of the rainbow, even more shades than any colored gemstone.  The most rare diamond colors are red, pink, green, and blue.  Diamonds that display enough of a hue, or nuance of color to be desirable, are called fancy-colored diamonds.

The absence of color in diamonds is most rare and highly prized.  Most diamonds mined in nature have traces of yellow, some brown or gray.  Color, in diamonds is caused by minute traces of other elements, such as nitrogen (yellow) and boron (blue). 

Color is an important factor to beauty, rarity, and value because it is something a consumer can see without the aid of equipment.  The range of color most often represented and sold in jewelry stores are:

AGS grades 0 - 3.0 
GIA grades D - J

The colorless to the near colorless ranges.

AGS Colorimeter GIA Color Scales

The most rare grades, AGS 0 - 1.0 or GIA D - F, are considered to be in the colorless range and are the most valuable.  Diamonds graded AGS 3.5 - 4.5 or GIA K - M show a visible yellow, brown, or gray body color face-up.  Yet when these diamonds are well cut and set in yellow gold, they can be beautiful, brilliant, and represent good value. 

How Color is Graded

The diamond in question is compared to a set of master diamonds under a balanced light source.  Once the color match is made, an Alpha Numeric grade is assigned. 

Terminology

Blue-White

According to the Federal Trade Commission, only a diamond that is either bluish in color or colorless may be referred to using this term, therefore it is NOT applicable in most situations.

Fluorescence

A unique quality possessed by some diamonds to give off light when exposed to specific ultraviolet wavelengths.  It is an unusual quality that can enhance the appearance of a diamond in certain lighting conditions.  When fluorescence is very strong, however, the diamond may have an oily appearance that detracts from the beauty and value.   Only 50% of all gem-quality diamonds fluoresce, and only 10% strongly fluoresce.  When gem-quality diamonds fluoresce it is a bluish color.

Color and its Relation to Value

Color is an important factor in the purchase of a diamond and can effect the price by 5 - 20% on each increment of the diamond grading scale.  Since it is a factor a consumer can see, careful examination of each diamond under consideration is advised.  This is especially true when considering a fancy-shaped diamond. For example, an emerald shaped diamond reveals the body color and clarity much more than other shapes, where even a slight trace of color may be easily discerned.

CLARITY
Clarity: Diamonds were formed millions of years ago, deep within the earth under enormous heat and pressure. Consequently, most diamonds contain tiny marks, known as inclusions, which make each stone unique. The inclusions don't necessarily weaken the stone and when they don't interfere with the passage of light they do not affect its beauty. The fewer and smaller the inclusions, the more valuable a diamond will be.
Definition

The presence or absence of inclusions within the diamond and blemishes on its surfaces.  These slight birthmarks of nature generally do not affect the beauty of a stone, but they almost always affect its price.  Truly flawless diamonds are extremely rare and very valuable. 

How Clarity is Graded

A diamond's clarity grade is determined through examinations by an experienced grader, using a 10x magnification and also the assessment of the trained unaided eye.  A lab utilizes binocular microscopes for clarity grading, which provides the best optical, lighting, and viewing conditions.

When a diamond is examined, the size, type, position, number, color, and relief of clarity features are observed.  The diamond is always viewed with the trained unaided eye to check for relative visibility of clarity features.

Terminology

Perfect	Only a diamond that is flawless, colorless and well cut can be referred to by this term.  Therefore, it is NOT applicable in most cases and should be avoided.
Carbon- Spot	Carbon-Spot Diamond is 99.95% pure carbon, and 25 different mineral inclusions or small crystals, have been found within diamond.  These are not carbon spots, but rather small crystals that were trapped within the host diamond as it was forming in nature.  The most common mineral found within diamond is diamond.

Clarity and its Relation to Value

Clarity is a rarity factor that affects diamond value by 5 - 20% for each increment on the diamond grading scale.  In most diamonds purchased by consumers, the diamond has few to no features noticeable to the unaided eye.  You simply cannot see the subtle differences in clarity without magnification.  Most consumers opt for a less rare clarity grade, and select a rarer color and larger carat weight.

CARAT WEIGHT
Carat weight: A diamond's weight is the simplest of its characteristics to measure and from the earliest times it was used to calculate a diamond's value. Diamond weight is measured in carats, and each carat is divided into 100 points A carat is equal to one-fifth of a gram.
Definition

A measurement of weight used in determining rarity in evaluating a diamond.

The term Carat is derived from the ancient Carob Seed from the locust tree, used as a medium of exchange on early pan-balances.  In the early 1900's the Metric Carat was established. 

1 Carat = .2 Gram

There are 100 Points to a Carat.  The AGS uses the international diamond standard tolerance for rounding which is 1/10 point. 

Example .999 = 1.00 Carat.

Terminology

Full Carat	A diamond that weighs or rounds to 1.00 Carat.
Light Carat	A diamond that weighs slightly less than 1.00 Carat.  Example .96 - .99.  This IS NOT a full Carat.  Diamonds in this range are priced less.
Magic Sizes	Diamonds that weigh in exactly at or greater than a major size category.  Each major size category marks a greater degree of rarity, and commands a higher price.

Carat Weight and its Relation to Value

Carat weight usually has the greatest impact on value, based on rarity.  Most people aspire to own at least a 1-carat diamond.  DeBeers states, "fewer than 1% of all women will ever own a 2 carat or larger diamond". 
Most fancy-shaped diamonds are elongated in shape and appear larger than a comparable round brilliant diamond.  In addition, fancy-shaped diamonds are priced less than round brilliants.