Grading :
In the last two decades there has been a proliferation of certification, not only for diamonds but for gemstones as well. There are a number of reputable laboratories which grade and provide reports on gemstones.
International Gemological Institute (IGI), world's largest independent laboratory for grading and evaluation of diamods, jewellery and colored stones.
Gemological Institute of America (GIA), the main provider of education services and diamond grading reports
American Gemological Society (AGS) is not as widely recognized nor as old as the GIA but has a high reputation.
American Gem Trade Laboratory which is part of the American Gem Trade Association (AGTA) the largest trade organization of jewelers and dealers of colored stones
American Gemological Laboratories (AGL) which was recently taken over by "Collector's Universe" a NASDAQ listed company which specializes in certification of many collectables such as coins and stamps
European Gemological Laboratory (EGL).
Gemological Association of All Japan (GAAJ), aka Zenhokyo, the preferred lab within Japan, also very active in the gemological research
Gemmological Institute of Thailand (GIT) is closely related to Chulalongkorn University, and has a good reputation for their gemmological research
Asian Institute of Gemmological Sciences (AIGS), the oldest gemological institute in South East Asia, involved in gemological education and gem testing
Swiss Gemmological Institute (SSEF), founded by Prof. Henry Hänni, offering a high scientific standard, and focusing on coloured gemstones and the identification of natural pearls
Gübelin Gem Lab, the traditional Swiss lab founded by the famous Dr. Eduard Gübelin. Their reports are widely considered as the ultimate judgement on high-end pearls, coloured gemstones and diamonds
Each laboratory has its own methodology to evaluate gemstones. Consequently a stone can be called "pink" by one lab while another lab calls it "Padparadscha". One lab can conclude a stone is untreated, while another lab concludes that it is heat treated [3]. To minimise such differences, seven of the most respected labs, i.e. AGTA-GTL (New York), CISGEM (Milano), GAAJ (Tokyo), GIA (Carlsbad), GIT (Bangkok), Gübelin (Lucerne) and SSEF (Basel), have established the Laboratory Manual Harmonisation Committee (LMHC), aiming at the standardisation of wording on reports and certain analytical methods and interpretation of results. Country of origin has sometimes been difficult to find agreement on due to the constant discovery of new locations. Moreover determining a "country of origin" is much more difficult than determining other aspects of a gem (such as cut, clarity etc.)
Gem dealers are aware of the differences between gem laboratories and will make use of the discrepancies to obtain the best possible certificate [3]. One such example is to make use of the differences in country of origin: a sapphire from Kashmir, India (celebrated for its cornflower blue color) commands four times the price of the same stone from Sri Lanka and twice the price as a similar stone from Burma

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 gemstones. 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, variscite, 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 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. Special equipment, a faceting machine, is used to hold the stone onto a flat lap for cutting and polishing the flat facets.[8] Rarely, some cutters use special curved laps to cut and polish curved facets.

Gemstone color :
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.
As an 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