Clarity Grading Scale
• IF Flawless. No visible inclusions under a 10X loop.
• VVS Very, Very Slightly Included. Minute inclusions that are difficult to see using a 10X loop and are not visible at all to the naked eye.
• VS Very Slightly Included. Minor inclusions that are visible using a 10X loop, but still not visible to the naked eye.
• SI 1 Slightly Included. The inclusions are visible using a 10X loop and are visible with the naked eye.
• SI 2 Slightly Included. The inclusions are more easily visible using a 10X loop and more noticeable with the naked eye.
• I 1 Included. The inclusions are easily visible with the naked eye.
• I 2 Included. The inclusions are very obvious to the naked eye and have a severe negative effect on the over-all appearance, durability and value of the gem.
Mohs' Hardness Scale
The practice of defining the fragile nature of one gemstone compared to that of another was developed by the German gem expert Friedrich Mohs back in 1812 and is still in use today. Simply put, the Mohs' Hardness Scale is expressed in numerical values of one through ten, with one being the most easily scratched and ten being the most scratch resistant. Most gemstones are compared to the Diamond which has the value of ten.
Carat Verses Size
One of the most common mistakes made by gemstone buyers is selecting a gem based on carat weight rather than size. Most people are familiar with diamonds, and have a good idea of the size of a 1 carat brilliant-cut diamond. But if you buy a 1 carat sapphire you are likely to be disappointed if you expect it to be the same size as your 1 carat diamond. The sapphire will appear substantially smaller.
There are two reasons why this is so. One reason is that sapphire is a denser material than diamond. Since the different gemstone varieties have different chemical compositions, they vary with respect to their density or specific gravity. Diamond is not a particularly dense material as far as gemstones go, and a number of gemstones have a higher density, including sapphire, ruby, spinel, garnet and zircon. On the other hand, some gems such as tourmaline, emerald and quartz have a substantially lower density than diamond, and will appear larger in the same carat weight.
A second reason why a 1 carat sapphire will appear smaller than a 1 carat diamond has to do with cutting style. Diamonds are cut to maximize brilliance, while colored gems are cut to maximize color. Diamonds have a larger "face-up" size, since they are cut with a large crown and a small pavilion. Colored gems like sapphires are cut with different proportions: smaller crowns and larger pavilions. This approach results in a richer color.
If you are careful to check the size when purchasing a gemstone, you will never go wrong, since a 6 mm round stone is always 6 mm, regardless of its weight. A 6 mm round ideal-cut diamond will weigh about 0.80 carats. But a 6 mm round sapphire may weight anywhere from 1.2 to 1.4 carats.
If you are buying a colored gem to fit a jewelry setting, it is especially important to buy by size rather than carat weight. A jeweler can make a custom setting to fit any size gem, but manufactured jewelry settings come in standard or calibrated sizes. You will find calibrated sizes for many different gemstone shapes, including round, oval, pear, marquise, octagon, square and heart.
Specific Gravity Chart
Specific gravity is a way to express the relative density of a gemstone. It is measured as the ratio of the density of the gemstone to the density of water. It is expressed as a number which indicates how much heavier the gemstone is compared to an equal volume of water.
Scientifically, specific gravity is defined as a ratio of the mass of a given material to the mass of an equal volume of water at 4 degrees centigrade. Most gemstone substances are two to four times denser than an equal volume of water. Specific gravities are expressed in decimal numbers, for example, 4.00 for corundum, 3.52 for diamond, and 2.72 for quartz. Zircon, one of the densest of all gemstones, may have a specific gravity as high as 4.73!
Specific gravity is an important tool in gemstone identification. But it is useful for the gem buyer as well as the gemologist. A 1 carat stone with higher specific gravity will be smaller than a 1 carat stone with lower specific gravity. So don't be surprised when the sapphire you buy is smaller than your diamond. That's because sapphire has a higher specific gravity. On the other hand, the 5 carat tourmaline you buy might be larger than you expected, because tourmaline has a fairly low specific gravity.
|Hematite||5.12 - 5.28||Iron Oxide||Trigonal|
|Pyrite||5.00 - 5.20||Iron Sulphide||Cubic|
|Spessartite Garnet||4.12 - 4.18||Manganese aluminum silicate||Cubic|
|Ruby||3.97 - 4.05||Aluminum oxide||Trigonal|
|Star Ruby||3.97 - 4.05||Aluminum oxide||Trigonal|
|Sapphire||3.97 - 4.05||Aluminum oxide||Trigonal|
|Star Sapphire||3.97 - 4.05||Aluminum oxide||Trigonal|
|Zircon||3.93 - 4.74||Zirconium silicate||Tetragonal|
|Rhodolite Garnet||3.85||Manganese aluminum silicate||Cubic|
|Chrysoberyl||3.70 - 3.78||Beryllium aluminum oxide||Orthorhombic|
|Alexandrite||3.70 - 3.78||Beryllium aluminum oxide||Orthorhombic|
|Tsavorite Garnet||3.57 - 3.73||Calcium silicate||Triclinic|
|Spinel||3.54-- 3.63||Magnesium aluminum oxide||Cubic|
|Diamond||3.50 - 3.53||Carbon||Cubic|
|Topaz||3.49 - 3.57||Aluminum fluosilicate||Orthorhombic|
|Mystic Topaz||3.49 - 3.57||Aluminum fluosilicate||Orthorhombic|
|Tanzanite||3.35||Calcium aluminum silicate||Orthorhombic|
|Peridot||3.28 - 3.48||Magnesium iron silicate||Orthorhombic|
|Chrome Diopside||3.22 - 3.38||Calcium magnesium silicate||Monoclinic|
|Kunzite||3.15 - 3.21||Lithium aluminum silicate||Monoclinic|
|Jadeite||2.90 - 3.03||Calcium magnesium iron silicate||Monoclinic|
|Prehnite||2.82 - 2.94||Calcium aluminum silicate||Orthorhombic|
|Aquamarine||2.68 - 2.74||Aluminum beryllium silicate||Hexagonal|
|Emerald||2.67 - 2.78||Aluminum beryllium silicate||Hexagonal|
|Beryl||2.67 - 2.78||Aluminum beryllium silicate||Hexagonal|
|Rose Quartz||2.65||Silicon dioxide||Trigonal|
|Onyx||2.65 - 2.91||Silicon dioxide||Trigonal|
|Smoky Quartz||2.65||Silicon dioxide||Trigonal|
|Mystic Quartz||2.65||Silicon dioxide||Trigonal|
|Pearl||2.60 - 2.85||Calcium carbonate||Orthorhombic|
|Chrysoprase||2.58 - 2.64||Silicon dioxide||Trigonal|
|Tigerâ€™s Eye||2.58 - 2.64||Silicon dioxide||Trigonal|
|Chalcedony||2.58 - 264||Silicon dioxide||Trigonal|
|Moonstone||2.56 - 2.59||Potassium aluminum silicate||Monoclinic|
|Jasper||2.50 - 2.90||Silicon dioxide||Trigonal|
|Bolder Opal||1.98 - 2.50||Hydrous silicon dioxide||Amorphous|
|Fire Opal||1.98 - 2.50||Hydrous silicon dioxide||Amorphous|
|Opal||1.98 - 2.50||Hydrous silicon dioxide||Amorphous|
|Amber||1.05 - 1.10||Oxygenated hydrocarbon||Amorphous|
Important Note About Weight And Color
• Weight and size of gemstones may vary slightly from piece to piece due to difference in calibration of weighing and measuring scales.
• Color my not appear exactly as in real life due to variation between computer monitor resolution and color settings.
Lab Created Gemstones
Labs created gemstones are not imitations. For example, diamonds, rubies, sapphires and emeralds have been manufactured in labs to possess identical chemical and physical characteristics to the naturally occurring variety. Synthetic (lab created) corundums, including ruby and sapphire, are very common and they cost only a fraction of the natural stones. Smaller synthetic diamonds have been manufactured in large quantities as industrial abrasives. Larger synthetic diamonds of gemstone quality, especially of the colored variety, are also manufactured. Whether a gemstone is a natural stone or a lab-created stone, the characteristics of each are the same. Lab-created gemstones tend to have a more vivid color to them, as impurities are not present in a lab, so therefore do not affect the clarity or color of the stone. Most all large rubies and sapphires with a VVS clarity rating are lab-created. However, natural gemstones are still considered more valuable on average due to their relative scarcity, however the cost will be substantially more and with some gemstones, they are not to be had at any cost. Example: A natural Ruby or Emerald with clarity of VVS or better weighing 10 cts. or more could not be purchased at any price.
Most minerals contain visible traces of their genesis, perhaps tiny crystals of other minerals that were caught up in the growth of the larger host crystal or formed simultaneously as it grew internal fractures that have been partially healed during growth or traces of earlier growth stages marked by zoning. Gemologists use the word inclusions to describe these and other internal phenomena.
Heat can improve a gemstoneâ€™s color or clarity. Many aquamarines are heated to remove yellow tones, change the green color into the more desirable blue or enhance its existing blue color to a purer blue. Nearly all tanzanites are heated at low temperatures to remove brown undertones and give a more desirable blue/purple color. A considerable portion of all sapphires and rubies are treated with a variety of heat treatments to improve both color and clarity.
Radiation is used to change or enhance the color of a gem. Very low amounts are used as to make it safe and no trace of radiation will linger in the gemstone.
Color is the most obvious and attractive feature of gemstones. The color of any material is due to the nature of light itself. Daylight, often called white light, is actually a mixture of different colors of light. When light passes through a material, some of the light may be absorbed, while the rest passes through. The part that is not absorbed reaches the eye as white light minus the absorbed colors. A ruby appears red because it absorbs all the other colors of white light (blue, yellow, green, etc.) except red.
The same material can exhibit different colors. For example ruby and sapphire have the same chemical composition (both are corundum) 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, the latter called "Padparadscha sapphire".
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.
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.
Cutting And Polishing
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 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. Grinding wheels and polishing agents are used to grind, shape and polish the smooth dome shape of the stones.
Gems which are transparent are normally faceted, a method which shows the optical properties of the stone's interior to its best advantage by maximizing reflected light which is perceived by the viewer as sparkle. There are many commonly used shapes for faceted stones. The facets must be cut at the proper angle, 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. The faceting machine is used to hold the stone onto a flat lap for cutting and polishing the flat facets. Rarely, some cutters use special curved laps to cut and polish curved facets.
Grading Of Gemstones
There is no universally accepted grading system for gemstones. Diamonds are graded using a system developed by the Gemological Institute of America (GIA) in the early 1950s. Historically, all gemstones were graded using the naked eye. The GIA system included a major innovation: the introduction of 10x magnification as the standard for grading clarity. Other gemstones are still graded using the naked eye (assuming 20/20 vision).
A mnemonic device, the "four Cs" (color, cut, clarity and carats), has been introduced to help the consumer understand the factors used to grade a diamond. With modification, these categories can 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, cut is the primary determinant of value, followed by clarity and color. Diamonds are meant to sparkle, to break down light into its constituent rainbow colors (dispersion), chop it up into bright little pieces (scintillation), 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.
Physical characteristics that make a colored stone valuable are color, clarity to a lesser extent (emeralds will always have a number of inclusions), cut, unusual optical phenomena within the stone such as color zoning, and asteria (star effects). The Greeks, for example, greatly valued asteria in gemstones, which were regarded as a powerful love charm, and Helen of Troy was known to have worn star-corundum.
Historically, 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.
Aside from the diamond, the ruby, sapphire, emerald, pearl (strictly speaking not a gemstone) and opal have also been considered to be precious. 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.
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.
Gem prices can fluctuate heavily (such as those of tanzanite over the years) or can be quite stable (such as those of diamonds). In general per carat prices of larger stones are higher than those of smaller stones, but popularity of certain sizes of stone can affect prices.
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