Carats measure the actual weight of a diamond (one metric carats is equal to 0.2 grams). As the weight of a diamond can have a significant impact on its price, they are usually weighed to the thousandth of a carat.
The color of a diamond can range from no color (D, highest quality) to yellow color (Z, lowest quality), with the exception of fancy colored diamonds, which can be found in many colors including blue, pink, yellow and chocolate.
The clarity of a diamond refers to the presence or absence of blemishes (external faults; scratches or pits) and inclusions (internal faults; knots). Diamonds free of imperfections are the most valuable. There are 11 grades of clarity, ranging from FL (flawless) to Is (imperfect).
The cut of a diamond determines its shape. A brilliant round diamond (the classic cut) has 58 facets. The quality of proportions, symmetry and polish determine the value of a cut diamond.
Once a diamond is chosen, it is usually set within gold (yellow, white, rose) or platinum, which also impacts the cost of and quality of a ring. If looking for an engagement ring (note, I'm not hinting at anything - only right hand rings for me) one of the most famous engagement and anniversary ring retailers is Tiffany & Co. Checkout the Tiffany & Co. Diamond guide for more information on these gorgeous gemstones.
Who could say no to a turquoise box?
Another popular option (at least in the NY Metro area) is to visit the Diamond District, where you can pick out a ring a la carte, choosing every aspect of the ring from stone to setting.