"The most popular times to buy an engagement ring are Valentine's Day and Christmas," says Betty Sowarsh, vice president at Bridge Jewelry Co. Whatever the timing, Sowarsh advises that you invest not only dollars, but sense and buy the best quality diamond you can afford. To determine quality, buyers should consider these traits: cut, color, quality and size.

• The cut is the only human-controlled factor. The cut determines how the diamond reflects light. An ideal cut has the shapes and mathematical proportions that make a diamond shine with maximum brilliance.

• A diamond should be near colorless. The less color, the more valuable the diamond. The Gemological Institute of America (GIA) has a grading scale that ranges from D to Z, with D being colorless and Z being the most yellow. Sowarsh recommends buying a diamond in the D through I range.

• A high-quality diamond should have few flaws. The GIA also grades the clarity of a diamond; a jeweler can tell you which ones are best.

• The size of a diamond is determined in carats. A carat is a standard unit of weight for diamonds equivalent to 200 milligrams. Carats can also be measured in points, with one carat equaling 100 points.

"The average size now is one carat," Sowarsh said. "Every girl likes to say, 'I have a carat stone.'"

Sowarsh has noticed a few trends among her buyers.

"Right now, the princess cut is very popular," Sowarsh said, referring to the square-shaped cut. Diamonds are made into standard shapes: round, princess, oval, marquise (football-shaped) and emerald (rectangle).