One of the most popular times to "pop the question," is Valentine's Day. If you're eyeing a diamond for your sweethart, make sure you're familiar with the four Cs of diamond value -- cut, clarity, carat and color.

Cut: This refers to how the diamond has, literally, been cut -- its geometrical shape. Some popular shapes include round, oval, emerald, marquise, princess, pear and heart. Beware, men: You might want to get a feel for what your future bride might want, cut-wise. To the untrained eye, this is the most obvious "C" -- except for maybe carat.
Clarity: Sure, diamonds look clear enough, but most have slight flaws, or inclusions. They're graded for clarity, the best being an F, which is flawless -- and extremely rare. The good news is that you can't even see most inclusions without some sort of magnification. But if you can plainly see a white spot or other blemish in a diamond, it's a good sign to stay away from it.
Here are the grades of diamond clarity:

F	VVS	VS	SI
Flawless	Very Very Slight	Very Slight	Slightly Included

If you have a friendly clerk, perhaps he or she will let you look at diamonds with magnification to see if you can detect inclusions.
Carat: If you've ever made reference to the size of a woman's "rock," you were referring to the carat, or weight, of her diamond. One carat is equal to about 200 milligrams. It's also a good idea to get a feeling from the woman in your life about how big of a rock she'd like to wear. Contrary to popular belief, not every girl wants the biggest diamond her future husband can afford.
Color: In the case of diamonds, the less color the better. The more colorless your diamond, the more sparkly. The color scale runs from D to Z, beginning with Icy White -- the color, or lack thereof, of the most expensive diamonds -- and ending with a light yellow.

Now that the tough part is over, here are some diamond factoids from the Gemological Institute of America:

Did you know ...?

Diamond is the hardest natural substance on earth.
In their pure state, diamonds are colorless.
Blue and pink are the rarest-colored diamonds.
Yellow and brown are the most common-colored diamonds.
Diamonds are virtually fireproof. To burn a diamond, it must be heated to 1292 degrees Fahrenheit. (The typical house fire reaches a temperature of approximately 1100 degrees.)
Only one polished diamond out of 1,000 weighs more than one carat.
The word carat comes from the Carob Mediterranean tree, whose seed was used for centuries as the standard of weighing precious stones.
1 carat = .2 grams or .007 ounces.
The largest rough diamond, discovered in 1905, is the Cullinan diamond, weighing in at 3,106 carats (2.8 pounds)!

• Read more about diamonds from HowStuffWorks.com.