Grading Scale for the Clarity in Colored Gems
The value of colored gems is based on the 4 Cs of color, carat weight, cut and clarity. Clarity related to price is not quite as critical in color as it is in diamonds. It is important that colored stones are eye clean. Eye visible inclusions have a significant deduction on the value while stones above VS clarity do not generally increase in value whereas diamond will increase another 25% above VS clarity. Colored stones with small eye visible inclusions can drop in price up to 30% while obvious inclusions can drop the price from 50% to 75% depending on the gem specie.
There are several tools for identifying inclusions in colored gems. The 10 power jewelers hand held loupe or a 2 to 5 power visor are excellent for locating inclusions. After locating inclusions use your naked eye to determine whether the inclusions are eye visible and their affect on brilliance and the beauty of the gem. For those with good close-up vision the best tool is a strong led flashlight. I currently use a Maglite XL 200 on all buying trips. To use the light you place it close to the gem perpendicular to your eye and you will see it lights up the interior of the gem showing all inclusions. You can combine this technique with a visor for enhanced effect.
Typical inclusions are fractures which show up mirror like, needles, crystals and silk. Silk, because of the small size is one of the hardest to see and it can have a noticeable down grade on the brilliance because of how numerous the inclusions are. With enough silk we get stars (asterism) and catseyes. Many of these inclusions are identifying characteristics of different gems. Talking of silk again we can separate sapphire and garnet by the angle that the silk needles intersect.
When looking at rough gems it is sometimes necessary to combine magnification, perpendicular lighting with heavy liquids. The technique is the same but we immerse the gem rough in a liquid with a similar refractive index to the gem being tested. This technique makes the rough surface of rough gems more transparent. We use refractol for quartz and gems with lower refractive index and methylene iodide for higher refractive index.
Some gems like emerald, certain locales of gems like Tanzanian spessartite, demantoid garnets and rubellite tourmaline will almost always have eye visible inclusions. The Gemological Institute of America uses a grading system that notes this by placing gems in categories of type 1 or gems that are commonly clear, type 2 which are gems that are often included and type 3 which are gems which are almost always included. This is applied to the clarity grading system with type 3 gems actually being graded about two grades higher than a similar type 1 gem. So an Si2 clarity is bumped up to a VS clarity as is the case with type 3 emerald so a Vs clarity emerald will still have eye visible inclusions whereas a type 1 aquamarine will be absolutely eye clean as a VS clarity.
Hope this explains some things!