Gemstones General Information
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Gemstone General InformationA gemstone or gem (also called precious or semi-precious stone, a fine gem or jewel) is a piece of mineral, which in cut and polished form, is used to make jewelry or other adornments. There are several stones and organic materials which are not minerals such as Lapis Lazuli, Amber or Jet, but are still used for jewelry, and are therefore often considered to be gemstones as well.
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Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties and have aesthetic value. Rarity is another characteristic that gives value to a gemstone. Difference between semi-precious and precious stonesHistorically, gemstones were classified into precious stones and semi-precious stones. Because such a definition can change over time and vary with culture, it has always been a difficult matter to determine what constitutes precious stones In modern usage, the precious stones are diamond, ruby, sapphire and emerald, with all other stones being semi-precious. The distinction reflects the rarity of the respective stones in ancient times, as well as their quality: all are translucent with fine color in their purest forms, except for the colorless diamond. Other stones are classified by their color, translucency and hardness. Up to the discoveries of bulk amethyst in Brazil in the 19th century, amethyst was considered a precious stone as well, going back to ancient Greece. Even in the last century certain stones such as aquamarine, peridot and cat’s eye have been popular and hence been regarded as precious. The traditional distinction does not necessarily reflect modern values however. For example, while garnets are relatively inexpensive, a green garnet called Tsavorite, can be far more valuable than a mid-quality emerald. Use of the terms ‘precious’ and ‘semi-precious’ in a commercial context is misleading in that it deceptively implies certain stones are intrinsically more valuable than others, which is not the case. Nowadays such a distinction is no longer made by the trade. Many gemstones are used in even the most expensive jewelry, depending on the brand name of the designer, fashion trends, market supply, treatments, etc. Nevertheless, diamonds, rubies, sapphires and emeralds still have a reputation that exceeds those of other gemstones. Gemstone ValueThere may have been times when you have purchased or seen a stone that maybe looks similar to one another but one is much more expensive than the other. There are factors to each stone that determines its value and it is important to understand this information. There is no universally accepted grading system for gemstones. Diamonds are graded using a system developed by a company called the Gemological Institute of America (GIA) in the early 1950s. Historically, all gemstones were graded using the naked eye and most gemstones still are. There is a system noted as the “four Cs” (color, cut, clarity and carats), that has been introduced to help people understand the factors used to grade a diamond. With some modification, these categories can also be useful in understanding the grading of all gemstones. The four criteria carry different weight depending upon whether they are applied to colored gemstones or to colorless diamond. In diamonds, the cut is the primary determinant of value, followed by clarity and color. Diamonds are meant to sparkle, to break down light into its various rainbow colors (dispersion), chop the color up into bright little pieces, and deliver it to the eye (brilliance). In its rough crystalline form, a diamond will do none of these things; it requires proper fashioning and this is called “cut”. In gemstones that have color, including colored diamonds, it is the purity and beauty of that color that is the primary determinant of quality. For instance, when you see a rough ruby, untreated and uncut, it doesnt look very precious. Its value is a lot less than one that has been cut, polished, etc. Once the stone has been cut and polished, you can generally see any impurities within the stone which adds to or detracts from its overall value. If it has a lot of small breaks within the stone, it decreases its value as it is not fully transparent anymore. One with a lot less breaks will be more transparent and its colors will be more brilliant thus making it more valuable and a lot more expensive. Cut and polished gemstonesMost gemstones you see in modern jewelry have been cut and polished. A few gemstones are used as gems in the crystal or other form in which they are found. Most however, are cut and polished for usage as jewelry. The two main classifications are stones cut as smooth, dome shaped stones called cabochons, and stones which are cut with a faceting machine by polishing small flat windows called facets at regular intervals at exact angles. Stones which are opaque such as opal, turquoise, agate, etc. are commonly cut as cabochons. These gems are designed to show the stone’s color or surface properties as in opal and star sapphires. These stones are generally just polished. Gems which are transparent are normally faceted, a method which shows the optical properties of the stone’s interior. This is perceived by the viewer as sparkle. There are many commonly used shapes for faceted stones. But the facets must be cut at the proper angles, which varies depending on the optical properties of the gem. If the angles are too steep or too shallow, the light will pass through and not be reflected back toward the viewer and thus lowering its value and true beauty. Gemstone ColorGems which are transparent are normally faceted, a method which shows the optical properties of the stone’s interior. This is perceived by the viewer as sparkle. There are many commonly used shapes for faceted stones. But the facets must be cut at the proper angles, which varies depending on the optical properties of the gem. If the angles are too steep or too shallow, the light will pass through and not be reflected back toward the viewer and thus lowering its value and true beauty. The color of any material is due to the nature of light itself. Daylight, often called white light, is actually all of the colors of the spectrum combined. When light strikes a material, most of the light is absorbed while a smaller amount of a particular frequency or wavelength is reflected. The part that is reflected reaches the eye as the perceived color. A ruby appears red because it absorbs all the other colors of white light (green and blue), while reflecting the red. The same material can exhibit different colors. For example ruby and sapphire have the same chemical composition (both are corundum – a hard crystalline mineral consisting of aluminum oxide) but exhibit different colors. Even the same gemstone can occur in many different colors: sapphires show different shades of blue and pink and “fancy sapphires” exhibit a whole range of other colors from yellow to orange-pink. Without getting too technical, this difference in color is based on the atomic structure of the stone. Although the different stones formally have the same chemical composition, they are not exactly the same. Every now and then an atom is replaced by a completely different atom (and this could be as few as one in a million atoms). These so called impurities are sufficient to absorb certain colors and leave the other colors unaffected and thus portraying a variety of colors in the same stone. For example, beryl, which is colorless in its pure mineral form, becomes emerald with chromium impurities. If you add manganese instead of chromium, beryl becomes pink morganite. With iron, it becomes aquamarine. Some gemstone treatments make use of the fact that these impurities can be “manipulated”, thus changing the color of the gem. However, some gemstones are naturally created this way. Gemstone TreatmentWhat is meant by treatment on a gemstone? Generally this means that the stone has been heated and waxed Gemstones are often treated to enhance the color or clarity of the stone. Depending on the type and extent of treatment, they can affect the value of the stone.
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