Did you ever wonder what the words "diamond girdle" means, when you see it as a designation on the certificate or grading report of a loose diamond?
Well, the girdle is exactly what it implies (;-), the outer most circumference of the diamond.
Some diamond girdles are "polished", which is where the diamond cutter actually creates facets on the girdle.
Others, are left in their natural, or un-polished state, which appears grayish and matte in finish.
The best analogy I could give you, would be to describe the difference between polishing the edges of the lenses on your eye-glasses (rimless glasses for example..) or leaving them un-polished.
There is no effect on the beauty or shine of the diamond at all. A polished or un-polished girdle is purely subjective and personal.
Here is a 40X magnified picture of a polished girdle on a loose diamond.
As an added bonus, you can also clearly view one of the internal inclusions (blemishes) of the diamond at this level of magnification.