1. A method for generating a look-up table for use in clarity grading a gem comprising: collecting actual inclusion parameter data and an associated clarity grade for each of a plurality of gems; deriving from the actual inclusion parameter data and the associated clarity grades mathematical relationships which model interactions between clarity grades and combinations of inclusion parameters; and associating parameterized clarity grades with corresponding combinations of inclusion parameter value ranges based upon the derived mathematical relationships, so that for a set of input inclusion parameter values a corresponding parameterized clarity grade is provided.