The SI3 clarity grade has been in use for close to two decades.  It wasn't until about 1993 or 1994 that wholesale diamond reports started listing a trading price for this clarity grade.  There never used to be, officially speaking, such a clarity grade as SI3.  You see, there are diamonds on the market that have more inclusions than what an SI2 should have, but aren't as obvious as they would be in an Imperfect.  An SI3 is a stone that would be considered a low-end SI2.  Today the EGL, IGI and most laboratories grade stones including SI3 in their description.

After EGL - Los Angeles (European Gemological Laboratory) started to issue the SI3 grade, the Rapaport Diamond Report, or the Rap Sheet as it's known in the trade, added SI3 to its price list.  For those who are unfamiliar with the Rap sheet, it is the definitive (but no longer only) price guide for Diamonds. It is used widely by the wholesale as well as retail industry.