When you are planning to purchase a diamond it is important that you how diamonds are classified. Today, the 4Cs of diamond quality is the universal method for assessing the quality of any diamond, anywhere in the world. Moreover, there's the "Fifth C" : Certificate

The process that forms a diamond happens only in very rare circumstances, and typically the natural materials required are found only in small amounts. That means that larger diamonds are uncovered less often than smaller ones. Thus, large diamonds are rare and have a greater value per carat. For that reason, the price of a diamond rises exponentially to its size.
Diamonds and other gemstones are weighed in metric carats: one carat is equal to 0.2 grams, about the same weight as a paperclip. (Don’t confuse carat with karat, as in “18K gold,” which refers to gold purity.)

What Size Diamond Should I Buy?
First, determine your budget. One general rule of thumb when buying a diamond engagement ring is “two months' salary.” This is just a guideline; it’s not carved in stone
Deciding on carat size is really about striking a balance between size and quality. If you prefer larger jewelry items, and you are working within a budget, you can still find a larger diamond of excellent quality gem by selecting one which is graded slightly lower in terms of color and clarity.
Remember that slender fingers make small diamonds look bigger. If you have small fingers, a 1-carat diamond will look proportionately large — and an even larger stone may appear stunningly big!

Think about what sort of setting will hold the diamond, you’ll have to be sure that the setting you choose is made to fit the carat weight of your diamond.