When evaluating the visual performance of a diamond, you should look at it both with and without magnification. For best results, your light source should be diffused, such as a fluorescent or shaded lamp. Diffused natural daylight is one of the best sources for grading diamonds for brilliance. Do not use a bare light bulb or spotlight because while this type of lighting can accentuate a diamonds sparkle, it might also create dark shadows in the stone. Beware of jewelry store lighting which usually consists of halogen MR-16 bulbs or daylight bulbs that are designed to deceive by pumping high watt halogen streams into promotional goods to make them look like the crown jewels. Note that many jewelry stores use blue diochromatic filters to mask the body color of yellow diamonds (blue + yellow = white).

Well-cut, properly proportioned diamonds will be brilliant throughout the stone and will refract rainbow like colors, often even in a dimly lit room. Poorly cut diamonds will refract only white light, or appear dull and lifeless and often have dark or washed-out areas that allow you to see through the bottom of the stone. A diamond that has been cut too deep will often have a dark center called a nailhead. Diamonds that have been cut too shallow will often display a white circle around the table (top center flat facet) edge which is commonly referred to as a fisheye.