It is true that high magnification will often reveal minute flaws which
cannot be seen with the unaided eye. However, I believe it is a given that
any grading system is based upon the appearance of the specimen to the
unaided eye. We often say that "gem" means a "perfect" or "flawless"
specimen. But actually, a more accurate description of "gem" is "no flaws
detectable with the unaided eye, even on close inspection". A shell which
meets that standard is "gem", even though a fine scratch might be seen at
20x magnification.
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