Easily evaluate the cut of a diamond simply by viewing it through this fully corrected triplet proportion loupe. Hold the loupe towards a light source and compare the diamond against the diagram for the amount of red seen inside the stone. A well-proportioned brilliant stone will reflect red. If the stone is cut too shallow, you'll get a donut effect with red around the edges of the stone. A stone cut too deep produces a red circle in the center. Jewelers loupes.