Cut.

By the beginning of this century, the art of cutting had been so refined that a precise mathematical formula was developed. It called for most diamonds to be cut with 58 facets, each placed at a precise angle to the other. In a well-cut diamond, light enters through the table at the top, bounces inside between facets, then is reflected back out through the table, creating scintillation.

If a diamond is not cut properly, which is sometimes done to retain maximum weight, the light "leaks" out of the bottom facets, giving the diamond a dull appearance.

 Proper cutting and polishing contribute more to the beauty of a diamond than flawlessness or  color.  Well-cut diamonds are designed to maximize the scintillation (sparkle) and brilliance, regardless of shape.

For over five  hundred years, diamond cutters tried to perfect the most scintillating, the most fiery, the most brilliant cut. The “Old Miner” and the “European” cut, while attractive, fell way short of that goal.

The most popular shape today is the round “Brilliant” cut, also called the "Ideal” cut which was developed in 1919 by diamond cutter and mathematician Marcel Tolkowsky. 

It is possible for a stone to be flawless and absolutely devoid of body color, yet be lacking seriously in brilliance and fire, unless the proportions of its cut are good and its angles in relation to the plane of the girdle are reasonably close to ideal. If the pavilion angles are  too shallow, the result will be  a lack in brilliance, also known as a “shallow stone” or a  “fisheye.”

The following values are so important that they will be described in specific detail on any GIA

certificate.

The ideal angle for the pavilion (the angle of the facets between the point or culet to the girdle) of a brilliant-cut diamond should be 40 3/4 to 41 degrees. A variance  of one degree is detectable in appearance and a variance of more than two degrees is quite obvious. 

The crown angle (the angle of the bezel facets which extend from the table to the girdle) should be approximately 34 to 34 1/2 degrees. I would reject any diamond with a crown angle of less than 30 degrees or over 38 degrees.

The table % should be less than 64%, (acceptable range 52% to 67%) based on the (girdle) diameter. Failure to conform to this formula is immediately evident in appearance to the naked eye.  

The Brilliant or Ideal cut has 58 facets.

The crown (top) consists of 33 facets; (one table, eight star facets, eight bezel facets, and sixteen upper-girdle facets.)

The pavilion (bottom) consists of 25 facets; (one culet (which is sometimes omitted), eight pavilion facets, and sixteen lower-girdle facets.)

Since the turn of the century, cutters have developed a number of other cuts, called "fancy"  cuts or shapes.

The most popular fancy shape is the marquise, then the pear-shape, the oval, the emerald-cut and the heart-shape. 

Other fancy shapes include square-cuts, called Princess or Quadrillion; triangles called Trillions; and baguettes.

In addition, lasers are now being used to cut diamonds into virtually any shape imaginable, such as stars, animals, holiday symbols and more.

Faulty cutting is much more obvious to the eye than the difference of color or clarity grades. Experts estimate that some 65% of all diamonds sold range from fair to poor quality of cut.