The precious stones or gems include all the mineral species and varieties (including some rocks and some materials of vegetable or animal origin) that, subject to cutting or polishing, can be used in works of jewelry.
The value of these stones is determined by the intensity of their color, their purity, cut and size as well as by their rarity.
The color of a gemstone is its most important feature. To determine the color of a gemstone is necessary to examine the hue, tone and saturation. The most precious stones are those with a pure color and only slight shades of other colors in addition to the primary one. The tone is the color depth, which goes from colorless to black. The scale used for the description of the tone goes from "light" to "dark". The saturation, or purity of the color refers to the degree to which the gem is free from brown or gray hues. The most precious gems, which show little gray or brown, are often described as having "alive" color saturation.
The purity factor is the reference in the comparison of colored gemstones. Almost all gemstones contain inclusions (foreign substances that gem may contain), including the most valuable gems. The flawless gemstones are very rare and very expensive. Every single gemstone is composed of a combination of trace minerals, which create a unique set of signs or inclusions. The inclusions, however, do not diminish the beauty or desirability. Some varieties of precious stones, such as aquamarine and blue topaz, have fewer inclusions, while other gemstones, like emerald and ruby, tend to have higher rates of inclusions.
Unlike diamonds, precious stones do not have an ideal cut aimed at bringing out their beauty. To recognize the quality of the cut of a gemstone is necessary to consider several aspects. A good cut showcases the color, reduces the evidence of inclusions and gives a good overall symmetry and proportion. In a gem with a good cut the light reflects on the entire surface, the glaze is smooth, no scratches or nicks.
The carat weight of a gemstone, or its weight, not necessarily gives full information about its size. Different gemstones have different densities: a ruby is denser than a diamond, and then a 1-carat ruby is smaller than a diamond of 1 carat, although they have the same weight.
The most important precious gemstones are: ruby, emerald, sapphire.
After the diamond, the ruby is the most precious gem. Known for thousands of years, it was used to decorate crowns and scepters. The ruby is characterized by a red color with bright tones, although you can also find pink or burgundy varieties. The finest rubies are those bright red and marked by great clarity, rare quality to find.
The emerald is a precious stone used in the crowns of kings and embedded in the eyes of the idols of India and the Far East. In Buddhism it is considered one of the seven treasures and equated to wisdom. Relatively hard, very often presents numerous inclusions and fissures on the surface that make this stone quite fragile. An emerald gem is rated of high quality when it has the typical and vivid green color, a geometric cut and perfect high clarity (size could exceed 5 carats).
Sapphire is a corundum and you can find any color, although ideally it backs to the blue. Other colors are available like yellow, green, pink, purple, etc.. Sapphires are particularly appreciated for their high gloss and hardness; after the diamond, these gems are the hardest of all. Concerning the merit of the individual sapphires, the more a gem is brilliant and has an intense color, the higher is its value. Even the clarity has its importance: high-quality sapphires are always free of visible inclusions. The stones large in scale are extremely rare and this is reflected in their price; a sapphire being of more than 3 carats is already considered very rare.