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Length to width ratio  
A comparison of the length and width of the girdle outline on fancy shape diamonds. The ratio is found by dividing the length of the diamond by the width. The width is always stated as 1. Some length to width ratios are considered to be more appealing than others, but this is a matter of personal preference.
Pear 1.50 - 1.75 : 1
Marquise 1.75 - 2.25 : 1
Heart 0.98 - 1.02 : 1
Oval 1.33 - 1.66 : 1
Emerald 1.50 - 1.75 : 1
Radiant/Princess 0.95 - 1.05 : 1

Light leakage
The amount of light lost through a diamond when it has not been cut to perfect proportion and symmetry. If the diamond cut is too shallow or too deep, light that is refracted within the stone will seep through the sides resulting in a loss of brilliance and sparkle. Diamonds with a zero light leakage such as the Brilliant 10, will display a higher degree of fire, brilliance and scintillation. In effect, this means that light that enters the diamond will be refracted within the stone and then reflected back through the top again.

Loupe
A small magnifying lens used to examine diamonds. 10x magnification is the standard measurement tool for determining the clarity of a diamond.

Lower-girdle facet
The facets on the pavilion of a round brilliant cut found just below the girdle.

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