There is no generally acknowledged evaluation scheme for gemstones. The idea that some gemstones are precious and others are only semi-precious is a fairly modern idea dating back to the 19th century. Conventionally four gemstones are currently considered “precious”: diamond, ruby, emerald, and sapphire, due to their unique color, splendor, and extreme rarity. All other gemstones were placed in the "semi-precious stone" category. The first use of "semi-precious" can be traced back to 1858. Gems have been advertised as “precious” in order to increase sales in definite areas of the market. In fact, the conventional list of precious gemstones is much longer and contains some unexpected members like pearl and opal, which were considered to be precious. Amethyst was considered to be a precious gem until the middle of the 19th century. Pearls have been categorized as both precious and semi-precious at different periods of the history. There are several “semi-precious” gems that are much rarer than diamond, emerald, ruby, and sapphire. For instance, tanzanite is extraordinarily rare and is found only in Tanzania, but isn’t considered to be precious gem. A tsavorite green garnet is more valuable than a mid-quality sapphire. The classification of gemstones into precious and semi-precious groups misleads many people that precious stones are more valuable than semi-precious stones. Thus the US Federal Trade Commission has considered banning the two terms because they unnecessarily confuse customers.
Another approach is to separate gems into organics and inorganics. Organic gems are those which creation is associated with living organisms. Pearls are created by oysters and mussels. Amber is a kind of sap produced by a tree. Coral is made by tiny populations of animals in the ocean. Most gem stones are made from non-living materials and are called inorganic gems. Everything in the mineral world falls into the inorganic classification. These are created by minerals without the help of organisms. In order to classify a gemstone as a mineral in the U.S., it has to have been created in the earth. That’s why stones created in a lab, even though they may chemically and physically identical to those found underground, cannot legally be described as minerals in the U.S.
Many types of gemstones belong to groups or species which share a common crystal and chemical configuration. Although, gemologists distinguish 16 gemstone mineral groups, there are two main groups of importance in the study of colored stones. The first includes garnet. It is consisted of a number of minerals having the same crystal construction but with differences in chemical structure. Thus the garnet group has a number of closely related species. The second is the feldspar group, which includes amazonite and moonstone as varieties of different species. Not every gemstone variety belongs to a group. Many important gems, such as peridot, zircon, topaz and spinel, are exceptional species that don't share properties with any other kind of gem and actually so called stand-alone gem types.
Colored gemstones and diamonds are also graded by four qualities, known as the “four C’s”. They are as follows: cut, color, clarity, and carats. For a colored stone usually color is the most important factor in determining quality (however, with diamonds, cut is the major aspect). Highest values go to stones with pure hues and strong rich colors. Clarity is judged by reference to inclusions. The function of the cut is to display the gem’s inherent beauty to the greatest extent possible. A well cut gemstone is able to handle the play of light. Carat is the unit of measurement used to weigh diamonds and gemstones. One carat is equal to 200 milligrams or 0.20 grams. Gemstones are measured in size (mm).
Gemstones may also be categorized in terms of their "water". This is a recognized grading of the gem's luster, transparency, "brilliance". Very transparent gems are considered "first water", while "second" or "third water" gems are those of a lesser transparency.
Today we think of precious stones like diamonds, emeralds and rubies as the most valuable gemstones in the world. However, there are so many of them that are even rarer and more appreciated. There are over 130 species of minerals that have been cut into gems. Gemstones have charmed our imagination in the ancient times and continue to do so even now. It takes millions of years for crystals to form in nature, and only a portion of those will ever be found, mined, cut and sold as gems. All the gemstones ever discovered are treated in their many variations, in its singular glory, in its rough natural state and in its polished and cut renditions.
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