When evaluating a diamond for brilliance, you should look at it both with and without magnification. The 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. 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 quartz or halogen bulbs. These can deceive as they blast high-intensity streams of light into goods to make them look like the crown jewels.

Well-cut diamonds will be brilliant throughout the stone and will refract rainbow colors, even in indirect light. Badly cut diamonds will respond only to high-intensity white light, or appear dull and lifeless and have dark or washed-out areas. 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 edge which is commonly referred to as a fisheye.