"Diamonds are a girl's best friend" nothing could be closer to the truth.  Diamonds have a mystique like no other gemstone; they are the most classic of all gems.  "A Diamond is Forever" is known as the worlds best known Diamond advertisement.  It refers to a Diamonds icy beauty and timeless appeal and durability.  Diamonds have the fire of passion and the brilliance of joy.  For centuries Diamonds have held their mystic with their breathtaking beauty, brilliance and irresistible attraction.  Diamonds are a part of a couples love for each other,  a Diamond engagement ring is a symbol of a couples pledging of their everlasting love and devotion as they begin their lives together.  Diamonds are always associated with love.

 Diamond is the only gem made of one element, carbon.  Diamond is the most durable of gemstones, 10 on the Mohs scale for hardness.

 Diamond is the most expensive gemstone because they are rare and costly to mine. Many people believe that diamonds are only colorless.  In reality truly colorless diamonds are quite rare.  Diamonds come in natural colors of yellow, brown, purple, blue, green, black and red. Red is the rarest color.  These are called fancy colored Diamonds.   A well cut Diamond will make light perform in breathtaking reflections resulting in brilliance, fire and scintillation.  Brilliance is the white light reflections from the surface and inside of the Diamond.  Fire is the color flashes you see.  Scintillation (also called dispersion)  is the flashes of rainbow colors you see when the observer moves. A finished (faceted) Diamond is judged by 4 value factors to determine the quality and value, Clarity, Color, Cut and Carat weight.

Clarity:

This is a Diamonds degree of freedom of inclusions and blemishes.  This affects the Diamonds price.  Fewer than 1% of Diamonds are inclusion free.
 
Color:
 
Most valuable Diamonds display the least amount of color, unless fancy colored Diamonds. Majority of Diamonds have a slight yellow or brown body color, this is not noticeable till compared with another diamond of less color.  The less color seen the more valuable the Diamond.

Cut:

A Diamonds facet cut, the proportion and placement of the facets all affect the diamonds brilliance. If a Diamond is not well cut, the brilliance and fire will be very poor. Diamonds are faceted in round, pear, marquis, emerald, oval and other unusual shapes.

Carat:
 
Diamond prices tend to increase sharply as the carat weight goes up. Less than 1% of women ever own a Diamond that is over 1 carat. Being bigger is not always better, your budget and taste might go with a smaller Diamond.

Diamonds are weighed in units called carats.  A carat is measured in increments called points. 
One carat equals 100 points. 

1 Carat      =  100 Points
1/2 Carat  =  50 Points
1/4 Carat  =  25 Points

Two diamonds of equal carat weight can have very unequal value.   This depends on how the diamond was rated in cut, color and clarity.  The higher the ratings, the more rare and valuable the diamond.