Gemstones are generally divided into two categories: precious stones and semi-precious stones. This distinction dates back to the ancient Greeks and is based on the rarity of the stones in ancient times. There are four precious stones, also known as the cardinal gems. The cardinal gems include: the ruby, emerald, sapphire, and diamond. All four cardinal gems share similar characteristics- they are all translucent and fall at the upper end of the Mohs scale of mineral hardness. Aside from the cardinal gems, several others have been considered precious throughout history. Up until the 19th Century, Amethysts were considered precious stones. Pearls, which are not actually gemstones at all, have also been historically considered to be precious. The value of gemstones, both precious and semi-precious, is usually determined by evaluating their color, translucency, hardness, and size.
Rubies and sapphires are both varieties of the mineral corundum, or aluminum oxide. The ruby's deep red color comes from the presence of chromium in the aluminum oxide. While rubies can vary in shade, the most prized color is called pigeon blood-red. In ancient China, rubies were sometimes buried beneath the foundations of buildings, for good fortune. Ruby is the traditional birthstone for the month of July.
Sapphires, the other cardinal gem variety of the mineral corundum, can be any color other than red. The sapphire's variety of colors stem from the presence of iron or titanium in the corundum mineral. The most well-known color of sapphire is deep blue; however, possible colors include yellow, pink, and orange. Sapphire is the traditional birthstone for the month of September.
Emeralds are a variety of the mineral beryl. The deep green color of the emerald comes from the presence of chromium. Emerald is the traditional birthstone for the month of May.
Diamonds are a colorless variety of the mineral carbon. The diamond's name derives from the ancient Greek word for unbreakable. They score a 10 on the Mohs scale of mineral hardness, which is the maximum. Diamonds are most often found today in engagement rings. However, in ancient times, diamonds were used as wards against disease or accident, or worn to attract good luck. Diamond is the traditional birthstone for the month of April.
All of the colored precious stones are evaluated using the same standards as a diamond: color, cut, clarity, and carat weight. These standards are also known as the "4 C's." Color is the most important factor for evaluating the value of rubies, sapphires, and emeralds. Color is composed of three elements: hue, saturation, and tone. Hue refers to the actual color of the stone--red, blue, green, or otherwise. Saturation is the deepness or richness of the color. Tone is the lightness or darkness of the stone. When evaluating the color of a diamond, the highest rating of D is given only to diamonds which are completely colorless.
In recent years, processes have been developed to create synthetic gemstones. Cubic zirconia is a type of lab-created, imitation diamond. Synthetic gemstones do not share the flaws of their genuine, mined counterparts. However, they do not command the premium of genuine, high-quality natural gemstones.
