Diamonds are the world's most precious stone, formed over billions of years beneath the earth's surface. The enormous pressure and heat of the earth's mantle transforms carbon into diamonds, the most desirable and cherished gemstone. When you are looking for diamonds, you should educate yourself about a diamond's characteristics. These are what is referred to as the "Four C's": Color, Clarity, Cut and Carat. A fifth important characteristic you should take into consideration is also fluorescence.
Most diamonds have some tint to them, usually yellowish in tone. Truly colorless diamonds are extremely rare, and very expensive. The color of a diamond is rated on a scale, starting at D which is "colorless," down to J which is "near colorless." Just because a diamond has a higher color grade, such as I or J, does not mean it is a flawed diamond. Some people even prefer diamonds with a yellowish tinge because they throw off a lot of multi-colored spark and fire, whereas truly colorless diamonds can seem icy blue.
Clarity is determined by how many flaws, or "inclusions," are found inside the diamond when it is examined under magnification. A very flawed diamond will not sparkle as much as a flawless diamond will. Clarity is rated on the following scale: IF (internally flawless), VVS1-VVS2 (very, very slightly included), VS1-VS2 (very slightly included), S1-S2 (slightly included). Anything on this scale is likely not visible to the naked eye. Truly flawless diamonds, rated F, are very rare and expensive, and usually not found in most jewelry stores.
How a diamond is cut is the most important determining factor of whether or not the diamond will be attractive. A diamond cut rated "excellent" or "ideal" means that the cutter took every care to ensure that the diamond is cut to proportion. A poorly cut diamond will not sparkle or shine as much as a well cut diamond, because light doesn't bounce through the stone the way it should. A diamond with an ideal cut can even help to disguise flaws such as a yellowish color or lower clarity.
A diamond's "carat size" refers to its weight. One carat equals 0.2 grams. As diamonds increase in carat weight, price will also increase exponentially. While size is important, especially when it comes to an item like an engagement ring, you should never sacrifice quality for size. The more carats a diamond is, the more likely the diamond will show flaws such as a yellowish color or lack of clarity. Carat weight also relates to cut: a well cut diamond may even appear larger than it's carat weight due to the sparkle, whereas a poorly cut diamond may look smaller than its carat weight.
Fluorescence is an important characteristic of diamonds that isn't as widely discussed as other characteristics. Fluorescence is the blue light that is emitted by some diamonds under a black UV light. Some diamonds emit a strong blue glow, other diamonds emit a moderate glow, and some diamonds emit no glow at all. Fluorescence is complicated because it can either increase or decrease the desirability of a stone. For a colorless stone such as D or E on the color scale, fluorescence is not beneficial. However, for stones with a slight yellowish tinge such as an I or J color stone, a medium or strong blue fluorescence can help mask the yellow and make the diamond appear more white.