This relates to the marks of non-crystallised carbon called ‘inclusions’ found within the diamond. The inclusions vary in standards on the spectrum ranging from FL (flawless), SI1 to I3.
It is exceedingly rare for a diamond to be flawless, known as FL and is the highest level on the spectrum. SI1 is in the middle of the spectrum and it is recommended when buying a diamond that you should not purchase lower than this standard. I3 is the lowest as it includes the most inclusions and is least clear.
The clarity grade is determined by several factors including:The size of inclusions.How many inclusions.The positioning of the inclusions - inclusions found under the table affects clarity grade more so than inclusions found under the side facets.The visibility of the inclusions – the more visible the inclusions to the naked eye the less favourable the grade will be.