Paul Monfil's summary of standard grading is a good one.  In practice,
however, most dealers from time to time exaggerate the quality of their
shells - this is human nature: the trick is to learn which of the
dealers you propose to deal with do this habitually (no fingers pointed,
but if a list comes your way with 90% of the shells rated GEM, this is
highly suspicious!!), and which do it simply by accident or irregularly.
 In my experience, "GEM" shells, except for a few species of cowries and
a few cones that like to live in low-energy environments, are RARE - you
can almost always find something "wrong" ( a subjective concept: for
some, thin growth lines don't count unless there are a lot of them,
while others consider any tiny irregularity to be a "flaw", hence
undesirable in a natural object) on any shell, even without a
microscope.  WITH a good 10x lens, it is easily seen that there is no
such thing as a "flawless" shell.//  F+ is the most commonly used grade,
and is a rather subjective, "flexible" term in practice (nobody likes to
outrightly call a shell "F" since it is the kiss of death for selling it
unless you have a good low price on the thing!!): generally it means a
good quality shell - minor repair or other irregularity or two, such as
a couple of little lip chips or a small growth mark, which do not
interfere signifigantly with the natural beauty of the shell - ie, they
are easy to overlook unless one is quite particular. in general, if a
shell is rated F, it is not that pretty, but is good as a study shell,
or as a "starter" or "place-holder" for that species, while F+ specimens
look good in most collections.  F++ and Gem- are interchangeable, and
mean "ALMOST" GEM - only the most particular of collectors would spurn a
shell of this quality.  F+++ or worse, is a meaningless designation,
meaning F++/Gem-.
        Of course, grading is subjective, except for "GEM" which SHOULD refer
to a shell with NO visible imperfections or irregularities (if it does
not, it is a mistake) - one person's F++ might be another's F+, and some
dealers don't even use the F++/Gem- designation. Let the buyer be always
on their guard!!

From the soggy but blooming north (tulips and forsythia are in full
bloom, and the cherry trees are just starting - quite lovely!);
-Ross.