"gem" means flawless, an absolutely perfect specimen with no defects discernable, even upon close inspection.

"fine" means minor flaws, either natural (like scars from healed injuries) or post-collection damage (minor chips on the lip of a cone shell, or fine scratches on a cowrie) which do not greatly detract from the scientific or aesthetic value of the specimen, and which are not readily apparent except upon close inspection.

"good" means that major defects are present, but the principle identifying characteristics of the species are still intact.

"+" can be used to represent grades intermediate between the principle grades mentioned above. "fine +" means "better than fine, but not quite gem". "Fine++" would mean "virtually gem", for example one minute flaw that would be difficult to find, even upon close inspection. "Good +" means better than good, but not good enough to be listed as "fine".

Some dealers use additional "+" signs - "fine +++", which makes the point that the shell is virtually gem, but I personally think it gets a bit silly after two "+" signs. A shell that is "fine +++++++++" should just be listed as "fine ++". That makes the point adequately. Now and then someone uses "gem -", which really means the same thing as "fine ++".

The system is somewhat subjective, and one dealer's "fine++" is another dealer's "fine+". After a bit of experience, you learn just how meticulous a particular dealer is in his grading of specimen shells
(Answer by M. Paul Monfils via the Forum)

These tiny hairline scratches on the nacre can be either from natural or manmade causes. If the shell brushes against coral or falls onto sand or is buffeted by surf-tossed sand, such minute scratches can occur. Sand grains, being largely quartz and similar minerals, are much harder than the surface of the shell. But they can also occur from contact with mildly abrasive surfaces after collecting - sliding the shell across a tabletop, handling it with jewelry on your fingers, or even wiping the shell with a cloth that isn't soft enough or has some fine dirt or grit on it. Remember that the shell grading system is based on ordinary visual inspection - no flaws that can be detected upon close examination with the unaided eye. Special lighting may reveal some minute flaws that may not be seen otherwise; and magnification almost always will. Give me a good 20X microscope and I'll find flaws on any Cypraea, "gem" or not.
(Answer by M. Paul Monfils via the Forum)