Brightness and Contrast
Brilliance consists of two components, brightness and contrast. A well-cut diamond. will have brightness. As you move closer to the diamond you will see its contrast pattern becoming more apparent (areas of light and dark which compliment each other). A diamond that is cut too deep or too shallow has a tendency to allow light to leak from the pavilion of the stone. Diffused lighting does not produce fire or scintillation so it's good for judging color and clarity, and visible inclusions that may be masked by the lighting. If you're analyzing color, be sure to keep a white background in front of you, since a diamond picks up and reflects colors around it. Diamonds that are bright return light from their surroundings back to the viewer.
Fire
Dispersion of light that appears as flashes of rainbow colors is known as the fire in a diamond. A well-cut diamond, regardless of shape, should have a well balance of colored sparkle, especially in direct spotlighting. Several factors affect the amount of fire in a diamond; star facet length, lower girdle facet length, pavilion angle, facet junctions, the angle at which light enters the diamond, and the angle of the light rays as they exit the diamond.
Scintillation
When a diamond sparkles intensely as it moves, this is known as scintillation. Scintillation is well seen in a bright lit environment, where the natural fire of a diamond seems nonexistent. High-end jewelry stores often have bright lights reflecting from walls, small directional spots and additional pinpoint LED or fiber-optic lighting positioned inside the case. In such an environment, you can see sparkle just by changing your position relative to the diamond. If you turn the stone back and forth in your hand, a well-cut stone will put on a fireworks display of brightly colored and white sparkles, enhanced by shifting contrast.
Polish
Polish is the factor that affects how well light is able to pass through a diamond and is crucial to the brilliance of a diamond. Polish is graded by laboratories using the terms ideal, excellent, very good, good, fair and poor. It is important to choose a diamond that is graded with a good, very good, or excellent polish. Poorly polished facets may reduce the intensity of light reflected from, or refracted into and out of a diamond. Laboratories assess polish by examining the diamond, facet by facet, with a microscope.
Symmetry
Symmetry play an important role in a diamond. Symmetry means the exact reflection of form on the diamond's shape and arrangement. It also refers to the symmetry of the facet cuts in the diamond. Symmetry is the overall look of the diamond.
According to the AGS, with an ideally cut diamond, the stone must be placed at precise angles and contain precise proportions. This placement normally ensures an ideal balance between the maximum amount of brilliance and dispersion of light. Any divergence from these proportions will disturb the even distribution and dispersion of light within the diamond - this normally results in a loss of sparkle. However the GIA (Gemological Institute of America) does rate symmetry and there are seven ratings from Extremely Poor to Excellent.
Diamond Shapes
Round
The round brilliant cut diamond is the most popular and most researched diamond shape available in the diamond Industry. With innovations and technological advances diamond experts have used advanced theories of light behavior and precise mathematical calculations to optimize the fire, scintillation and brilliance in a round diamond. In addition to being the most popular and researched shape, a round diamond will typically give you more flexibility in terms of balancing cut, color, and clarity grades while still getting the fire, scintillation and brilliance one wants.
Princess
Square shape modified brilliant cut diamond, popularly known as Princess Cut Diamond. This is one of the most popular non-round diamond because of its unique cut that gives brilliance and fire to the stone makes it a favorite for engagement rings. The princess has 4 pointed corners and is traditionally square in shape. Also, princess-cut diamonds can vary greatly in how square or rectangular they are. To find the dimension of princess one should look for the length-to-width ratio in the diagram below. Here are length-to-width ratios for princess-cut diamond shapes that are pleasing to the eye.
For a princess diamond shape that is square, look for length-to-width ratios between 1 and 1.05. If you prefer more of a rectangular shape, look for length-to-width ratios greater than 1.10.
Emerald
Cut Cornered Brilliant Diamond also popularly known as Emerald Cut Diamond. What makes this shape different is its pavilion, which is cut with rectangular facets to create a unique optical appearance. Due to its larger, open table, this shape highlights the clarity of a diamond. Also, emerald-cut diamonds can vary greatly in how rectangular they are. If you'd prefer an emerald cut with a squared outline, look for an Asscher-cut diamond. To find the shape of emerald look for the length-to-width ratio in the diagram below. The length-to-width ratio will determine the diamond's outline, or what it will look like when viewed from the top.
For the classic emerald-cut shape, look for a length-to-width ratio between 1.30 and 1.40.
Asscher
Cut Cornered Square Brilliant Diamond, popularly known as Asscher Cut (Square Emerald cut) is octagon in shape. This beautifully unique shape is similar to the emerald-cut, except that it is square. Also, this shape has a pavilion that is cut with rectangular facets similar to the emerald-cut.
Oval
An oval diamond has beautiful brilliance that's similar to a round diamond. Oval diamonds are also very popular as their length can accentuate long, slender fingers. The length-to-width ratio will determine the diamond's outline, or what it will look like when viewed from the top.
For the most traditional oval diamonds, look for length-to-width ratios between 1.33 and 1.66.
Marquise
It is a boat shape cut. The shape of a marquise diamond gives an illusion of a much larger-looking diamond. This brilliant-cut diamond looks beautiful set with round or pear-shaped side stones. The length of the marquise makes fingers appear long and slender. The length-to-width ratio can be determine by the image below.
For the most traditional marquise-cut diamonds, look for length-to-width ratios between 1.75 and 2.25
Pear
This brilliant-cut diamond is also called a tear-drop as it has a single point on one side and rounded end on the other. This unique look of the pear shape helps make it a popular choice for a variety of diamond jewelry. If chosen an elongated pear shape, the length of the diamond creates a subtle slimming effect on the fingers. A lot of antique jewelry are made with this shape of a diamond.
For the most traditional pear-shaped diamond, look for a length-to-width ratio between 1.45 and 1.75.
Radiant
The name comes with its appearance. It radiates lot of brilliance with its facets. It is Cut Cornered Modified Diamond popularly called Radiant Cut Diamond. Trimmed corners are the signature of this diamond, and they help make the radiant-cut a popular and versatile choice for jewelry. A radiant-cut looks equally beautiful set with either baguette or round side-diamonds. Radiant-cut diamonds can vary in their degree of rectangularity. Radiant Cut diamonds do come in square and rectangular shapes.
For a radiant diamond shape that is square, look for length-to-width ratios between 1 and 1.05. If one prefers more of a rectangular shape, look for length-to-width ratios greater than 1.10.
Cushion
This unique shape has been popular for more than a century. Cushion-cut diamonds comes from its name cushion/pillow shape, have rounded corners and larger facets to increase their brilliance. Cushion-cut diamonds are available in shapes ranging from square to rectangular. A lot of antique jewelry is made using this shape.
For a cushion-cut diamond that is square, look for length-to-width ratios between 1 and 1.05. If you prefer more of a rectangular shape, look for length-to-width ratios greater than 1.15.
Heart
The heart is the ultimate symbol of love. The unique look of the heart-shaped diamond helps make it a distinctive choice for a variety of diamond jewelry. It is a choice for love.
For a more traditional heart-shaped diamond, look for length-to-width ratios between .90 and 1.10.