The loupe is a tool which jewelers use to look at gemstones “close up.” It is a small magnifying glass, usually 10X (i.e., ten power), with a black housing around the stem. The housing is black so as not to distort the color of the stone which is being examined. According to the FCT (Federal Trade Commission) diamonds must be graded using 10X magnification.  Incidentally, any flaw that cannot be seen under ten power magnification is considered not visible.