Cut -how well the stone is cut
The grading is done on the majority of these 3 results.
Color
Color is the single most important factor when evaluating colored gems. Basically, the more attractive the color seen, the higher the value. Bright, rich and intense colors are valued over those that are too dark or too light. Colors that are dulled by tones of black, gray or brown are regarded as less desirable. The colors seen should ideally remain attractive regardless of prevailing light conditions. Whether viewed indoors, outdoors, by day or by night, a gem should always remain beautiful.
The mixing of color hues into combinations, such as evergreen and turquoise and aqua and grass green in Emeralds is attractive and value enhancing.
Some people ask me if emeralds from Africa are a different color of green than emeralds from Brazil. They are not-when you see the two next to each other you cannot tell the difference. Emeralds from both countries come in all different shades of green. Being from one country versus the other makes no difference in the value of the emerald. You will see many different terms describing color on many different websites such as top green, best green, fine green, nice green, rich green, green-blue, blue-green, greenish blue, bluish green, etc.,. Although specific colors hues can affect the prices of gems, personal preferences are more important. Think of how the gem will look mounted, how it catches the light at different angles. Think about how you feel about the color-do you prefer a dark green or light green.
Clarity
Most gems contain tiny natural features called inclusions. Mostly microscopic in nature, they are most easily glimpsed under magnification. Inclusions that don�t interfere with the brilliance, sparkle and fire of a gem don't affect the value.
Many gems have tendencies to be more included than other varieties. For example, Emeralds are known to be far more included than Sapphires and this should be taken into account when making your selection.
The clarity of gems is determined by judging the amount and location of inclusions seen. Basically, the higher the clarity grade, the higher the value of the gem.
Emeralds are called Type III Gemstones and go by this type of Grading below concerning inclusions.
Type III Gemstones : Type III stones are almost always included and show eye-visible inclusions, but even specimens with obvious or prominent inclusions are often faceted for use in jewelry.
VVS, Type III - describes Gems that are as close to flawless as is found in the particular type III gemstone, a gemologist under favorable lighting conditions can see small inclusions (small feathers, light silk, light gardin, etc.) which are usually obvious when viewed with 10x magnification, but you will probably not readily see these inclusions to the unaided eye, except on larger stones.
VS, Type III - describes Gems that are very clean for the particular type III gemstone, a gemologist can see small sized inclusions (small fissures, light gardin, etc.) that is usually visible with the unaided eye without magnification. Larger Emeralds can have medium sized inclusions and still be graded VS Type III.
SI, Type III - describes Gems that are lightly included but the inclusions are not a negative for the particular type III gemstone, a gemologist can see moderate and/or numerous small inclusions (fissures, gardin, carbon, etc.) that are visible to the unaided or naked eye without magnification.
I1 to I2, Type III - describes Gems that have inclusions centrally located or numerous inclusions that are visible to the unaided or naked eye without magnification. Most Good quality Emerald will be in the I2 to I3 category.
I3, Type III - describes Gems that have very obvious inclusions that are very visible to the unaided or naked eye without any magnification. This grade of excessively included gem may have some durability problems.
Cut & Polish
Unlike Diamonds, colored gems possess variable optical properties and are not cut to a uniform ideal. A well-cut colored gem exhibits even color, a minimal number of inclusions, good brilliance and shows the majority of Carat weight when viewed from the top. A well-cut gem should also exhibit good symmetry and polish condition. Facets should be aligned straight in relation to the gem�s girdle and also to each other. Polish condition should be good with no visible surface pits and polishing lines.