Emerald, May's birthstone, has been prized for thousands of years for its lush green hues and rare beauty. Throughout the ancient world, emerald symbolized eternal hope, rebirth and the arrival of spring - and some cultures believed the gem rewarded its owners with love, intelligence and eloquence as well.

The ancients ascribed numerous magical and mystical properties to this most precious of green gems. In ancient Rome, for example, emeralds were believed to have a soothing effect on the soul. Modern scientists have since shown this myth to have some basis in fact: tests indicate that the human eye is more sensitive to green than any other color. Middle Age seers used emeralds to foretell the future, as well as to ward off evil spirits and cure ailments ranging from bad eyesight to infertility. The stone was also said to improve memory and bring great wealth to its wearer.

Derived from the Latin word for green, "smaragdus", emerald is also the traditional gift of choice for couples celebrating their 20th and 35th wedding anniversaries.

The finest emeralds have traditionally come from Colombia; both the Incas and Aztecs mined rich emerald deposits in the rugged Andes Mountains. But Russia's Ural Mountains also have produced top-quality gems. Brazil is by far the world's largest producer of emerald, with a wide range of quality. Other sources for the stone include Afghanistan, Australia, India, Pakistan, the United States, Zambia and Zimbabwe.

Emeralds come in a variety of light and dark shades of green - and often with subtle background hues of other colors like yellow, blue, brown or gray. Generally, the purer and richer the green, the more valuable the emerald. The gem ranks 7.5-8 on the Mohs hardness scale. This means that emerald, while relatively hard, can still be scratched, chipped or split fairly easily. Most emeralds have numerous flaws, or "inclusions", which weaken their structure. Flawless emeralds are exceptionally rare, and therefore command great prices (in some instances, higher than diamonds).

When shopping for emeralds, keep in mind that the gems are judged by size, color, clarity and cut (although because of the stone's penchant for inclusions, a lesser clarity is acceptable - as long as light is still reflected through the gem). Color is extremely important, and is broken down into three considerations: hue (the basic color of the stone, including any tints other than green); tone (the "depth" of color, ranging from "light" to "dark"); and saturation (the purity of the green and the level of other hues, if present).

Fissures, or cracks, are common in emeralds. Try to avoid those that penetrate too deeply into the stone, thus making it more susceptible to splitting.

Like most gemstones in the market today, emeralds are usually treated in some way to remove surface flaws and enhance color. The most common (and acceptable) technique is to oil the stone with a green-tinted oil to fill in surface cracks. The oil hardens and strengthens the stone, and improves its green color as well.