Jim Cordi has opined that "You really have to know your species to grade
them.".  I am afraid i must disagree: it is quite confusing to hold
different species up to different standards, based upon how often they
achive a very high quality condition - if a certain species almost
always lives in a rough neighbourhood, or if they live in sand and
therefore become eroded at the top with age, then they will simply
virtually never achieve an F++ to Gem standard - but what is wrong with
that?  Nature is a harsh mistress at times, and if some species nearly
always get the short end of the stick so far as the aesthetics of their
exoskeletons go, so be it: to use a different grading standard for these
can cause endless confusion: one orders a gem- shell only to find it is
objectively-speaking only on the high side of F+.  You ask the sender
what gives, and he/she/it replies that it is "gem- for the species,
which is almost never perfect" - nope: this is a recipie for disputes
and disappointments for certain.  All shells should be held up to the
same standard: this is the only fair way to proceed, for a variety of reasons.

From Thunderstormcentral,
Ross M.