(MONEY Magazine) – Keep this in mind as Valentine's Day approaches: Even though diamonds may have been Marilyn Monroe's best friend, plenty of women--as well as some men--prefer the warmth of a sapphire, an emerald or a ruby.

If you're in the market for one of these colored gems, you'll probably find the shopping experience intimidating. The typical retail price of a one-carat sapphire, emerald or ruby ranges from $450 to $1,800 for an average-quality stone and from $4,000 to $7,000 for a finer one, which can make these gems as costly as diamonds. What's more, these popular precious gems are trickier to shop for. Here's how to make your way:

CLEAR DIFFERENCES

If you've ever bought a diamond, you may already know a bit about gem pricing. Every diamond receives grades for four qualities--color, cut, clarity and carat--which are combined to figure the stone's value. (For more on buying a diamond, see the box on page 180.) Those four features also determine a colored gem's price, but jewelers don't assign easy-to-grasp letter grades to each. Instead, salesmen use vaguer descriptions such as "intense," "vivid" or "brilliant," which make comparison shopping a challenge.

And while most diamonds are distributed by one company, England's De Beers, colored gems are sold by numerous firms. "There's no control over the pipeline of colored stones," says Joseph Menzie, a New York-based dealer in colored stones. "So the laws of supply and demand set their price." For consumers, that means less predictable prices and more fluctuations year to year.

THE CRITICAL CS

With colored gems, the quality that has the greatest influence on the price is, not surprisingly, color--both the hue and what's referred to as saturation (whether the color is intense or dull). Stones with rare hues, such as a primary red with no trace of orange or a pure green with no yellow, are the most expensive. That's good news if you prefer a more common color, such as a dark blue sapphire. "If your tastes go against what connoisseurs like," says gemologist Craig Nass of New York City's H. Stern jewelers, "you'll get more gemstone for your dollar."

Clarity, perhaps the second most important feature, is mostly determined by the number and size of flaws or fissures, called inclusions. A good cut is one that brings out the richest color and allows light to play on the stone's surface and within. As for carats, keep in mind that larger gems, especially those over three carats, usually have higher per-carat prices because big high-quality stones are rare. So a six-carat dark blue sapphire may sell for $2,250 a carat, says Menzie, vs. $750 for a similar one-carat stone.

Of course, you don't need to stick with these three precious gems to get luscious color and high quality. There are more than a dozen less expensive blue, green and red alternatives. Some lesser-known gems like tanzanite (blue) and spinel (red) are even scarcer than rubies, emeralds and sapphires. Yet because they aren't in heavy demand, they're cheaper.

SHOP AROUND THE CORNER

Stick with a well-established jewelry store that specializes in colored gems. Unless you live near a regional chain such as Fortunoff (in the Northeast) or Ben Bridge (in the West), that probably means an independent mom-and-pop jeweler. Try to find one that can get a variety of stones to show you, is willing to substitute one stone for another in a setting you like and will let you return the gem if you're not satisfied. The salesperson should take time to explain why one gem costs more than another.

Here's more on what separates an expensive blue, green or red stone from a cheaper one, what to be alert for when you shop and what alternatives you should consider if you want to pay less.

FEELING BLUE

Sapphires are America's favorite colored gemstone. Fortunately, examples of the popular dark blue variety are plentiful, making them the most affordable of the three top-selling colored stones. An oval one- to 1.5-carat dark blue sapphire, most likely from Thailand or Australia, should cost from $450 to $750 a carat (depending on the clarity, color and size). You'll pay $2,200 to $4,000 for a more unusual one-carat Sri Lankan sapphire that's medium blue, a shade darker than royal blue and lighter than navy.

SHOPPING TIPS. Avoid a blue so dark that the sapphire appears nearly black. For a sapphire to sparkle, the stone should be light enough to see through. Inclusions are fairly common in sapphires, especially in larger stones, and they will lower the price. Don't worry, however, if your jeweler tells you that your sapphire has been heat-treated. Nine out of 10 sapphires are treated to permanently enhance their blueness.

BLUE ALTERNATIVES. No other stone can mimic the rich blue of a fine sapphire. But a popular substitute is tanzanite, a violet-blue stone that comes from the region near Tanzania's Mount Kilimanjaro and costs from $350 to $500 for a one-carat gem. When mined, a tanzanite is orange-brown; the crystal is heated to bring out a permanent blue hue. One advantage of tanzanite is that larger stones are less likely to have flaws (at least to the naked eye) than sapphires are. A flawless three-carat tanzanite would cost about $2,700, or $900 a carat. Two cheaper blue gems are the watery-blue iolite ($45 to $50 a carat) and the pale-blue aquamarine ($100 to $150 a carat).

GREEN LIGHT

The world's oldest source of emeralds is Egypt's Cleopatra's Mines, named after the queen who favored these green gems. Today, however, most retail emeralds come from Brazilian mines, which produce slightly dark, somewhat cloudy emeralds that cost between $625 to $825 for a one-carat stone. The most sought-after emeralds are Colombian, which are an intense grass green and sell for $3,500 to $5,000 for a one-carat stone.

SHOPPING TIPS. Of late, emeralds have become less popular as consumers have learned that jewelers often mask the gem's natural inclusions, says Laura Barringer, a colored-gemstone buyer with the Seattle-based jeweler Ben Bridge. While most inclusions are tiny fractures that make the stone look cloudy, others are pockets of gas, liquid or even another gemstone trapped within.

For centuries, emeralds have been soaked in oils or resins to fill fractures and make these inclusions less visible. The Federal Trade Commission requires jewelers to tell customers if this treatment has been used. The process shouldn't affect the price of most emeralds, says New York City dealer Menzie, but the fillers can dry out. So ask your jeweler if you need to have your gem retreated every two to five years, just as you must periodically restring pearl necklaces.

GREEN ALTERNATIVES. A flaw-free emerald lookalike is tourmaline. Green tourmalines, which come in a range of shades, run $150 to $250 a carat regardless of size. That makes them an excellent choice for today's trendy oversize cocktail rings. If you want a really inexpensive green gem for a large piece of jewelry, consider yellow-green peridot, which costs just $30 to $40 a carat.

BETTER RED

Because rubies are among the rarest of gemstones, they can cost more than emeralds, sapphires and even diamonds of the same quality. Expect to spend $1,500 to $1,800 for the most common retail ruby--a one-carat oval, somewhat hazy red stone from Myanmar or Thailand. Finer rubies (also from Myanmar) are a clear, medium red--think traffic lights or cherry Life Savers--and cost from $5,500 to $7,000 for a one-carat stone, says Peter Schneirla, a private jeweler in New York City.

SHOPPING TIPS. Because rubies and sapphires are composed of the same mineral, corundum, they share many qualities, including inclusions. Like sapphires, rubies are often heat-treated, which hurts the value only if the gem is of top grade. And what black is to dark sapphires, brown is to rubies. "In terms of quality, you would rather have a pinker ruby than a browner one," says Schneirla.

RED ALTERNATIVES. Consider two gems that never need treatment to enhance their color. Violet-red rhodolite garnets cost just $45 to $50 for a one-carat stone. A red spinel is a dead-on ruby double. In fact, some crown jewels of England and Iran, long thought to be rubies, are actually spinels. Spinels are rarer than rubies, according to Menzie, but cost just $600 to $750 for a bright-red one-carat oval.