After carat weight, clarity has the biggest impact on diamond prices. Diamond Clarity is symbolic of "purity" – the more flaws, the less valuable the diamond, but unlike emeralds, inclusions in diamonds are rarely "flaws" that result in breakage.
Medium clarity diamonds are just as brilliant as Flawless diamonds; even experts can not tell the difference between Flawless and SI1 diamonds without a loupe. SI2 is supposedly the borderline where inclusions become visible to your naked eye. If you can see an inclusion with your naked eye in normal light, from 14 inches (35cm) without having previously identified its position using magnification, then the stone is ‘I' for Imperfect or the European term – ‘ P' for Pique (PeeKay). However some young people with excellent eyesight may be able to spot a VS2 inclusion.
Precious Metals only stock diamonds in SI2 to Flawless, but we recommend SI1 and VS2 as being the best balance for quality and value.
Giving a grade based on inclusion quantity, size, placement and type is difficult; graders are human . Diamond grading reports are meant to resolve arguments between buyers and sellers, but ultimately they are just "expert opinions" under 10 times magnification with a loupe. Even GIA has given different grades for the same resubmitted stone.
EGL and EGL-USA introduced an SI3 grade in 1992. SI3 has been used in the dealer market for many years because of the big price difference between I1 and SI2. The World Federation of Diamond Bourses wants all labs to introduce SI3, but most refuse. Since 1992, most labs have softened their SI2 grades.
Grading reports include plots of inclusions (marked in red for internal and green for external features) and this is useful for identification. Often only the main "grade makers" are plotted, and additional inclusions are listed in comments: "pin points not shown" etc. A common comment is "Clouds Not Shown"; a cloud drawn on plots look so bad that no one would buy the diamond. Clouds are only a problem on SI1 and lower clarity's if no other inclusion is marked on the plot, i.e. the cloud is the grade maker; a big cloud may dull the diamond. It is rare for even I1 diamonds to be dulled by inclusions. Diamonds with SI1 (below left) and I1 (below right) Clarity, 10x magnification. 
|  |
