"Carat" is the standard measure used for diamond weight. 1.00 carat = .20 gram.

Diamonds are priced “per carat.” 

The heavier a diamond, the higher the price ‘per carat.’

Example: 

Consider two identical quality and shape diamonds: 

One weighs 1.00 carat and the other 2.00 carat. 

Assume the 1.00 carat stone price is $6,000 per carat = $6,000 for the diamond.  

The same quality 2.00 carat would cost $7,700 per carat = $15,400 for the stone.

Price per carat is directly related to weight and rarity. 

Mother Nature provides us with an adequate supply of diamonds weighing .75 carat and below, however larger diamonds are truly scarce and become progressively more rare and valuable as they get larger.  

To recover a single rough diamond weighing just 1.00 carat approximately 20 tons of earth must be processed... incredible but true!

A rough diamond loses 30 to 60 percent of its original weight in cutting depending upon the rough and the shape being produced. Therefore a 2.00 carat 'rough diamond' [on average] is required to produce a 1.00 polished gem.

The "price per carat" reality provokes cutters to produce heavier rather than more perfect diamonds and over 90% of all diamonds produced have unnecessary additional weight in order to produce heavier diamonds. 

Unfortunately manipulating the proportion to produce heavier diamonds also results in less brilliant diamonds with a smaller 'face to weight ratio'. 

To protect the value of your purchase you must consider carat weight relative to face size, cut and brilliance. 

A 2.00 diamond cut to maximize weight might be no larger than a perfectly cut 1.75 carat and will certainly be less brilliant and valuable.