The problem with shell grading is it’s direct connection to shell price.
As Marlo points out the market will only pay for the top most ratings.
Some dealers are driven to grade-up to make sales.  A few things can
happen.  The buyer can be forgiving of a false grade if they find they
like the shell enough to skip the hassle of returning it. Also there are
many collectors that don’t recognize certain types of flaws and feel they
got a shell accurately described.  Worse case scenario there’s a refund,
but I suspect this is relatively rare. I would imagine there is an
economic model from a business standpoint of what shells can be upgraded,
and in what ways, to result in the maximum profits.  Although I would
imagine this is a little deeper than many dealers think.  It probably just
comes natural to them.  I always wonder when I receive a flawed shell that
was claimed gem if the dealer doesn’t agree with my opinions on flaws, or
if the shell was up-graded to make the sale.  Rarely do I go through the
hassle of returning it.

I would imagine only through education could a true grading system based
on flaws be established.  Perhaps a book with photos detailing each type
of flaw.  The flaw scale wouldn’t be 1-10; it would be 1- however many
flaws there are.  Pretty simple, but it might ruin the innocence of some
collectors.

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