Luminescence (fluorescence) is the ability of a substance to emit light under the action of ultraviolet rays. Luminescence of diamonds is evaluated under ultraviolet irradiation with a wave length of 365 nm. The luminescence can vary in color (blue, yellowish, rose, greenish) and intensity. The luminescence is characterized by the following four grades:

• None
• Weak
• Moderate
• Strong

Luminescence color may also be indicated in the worksheet.

The luminescence grading is conducted with luminescent masterstones, in a darkened room with a light source with a wave length of 365 nm.

Blue luminescence may affect the diamond price: it is lower for diamonds of higher colors and higher for yellow stones. This can be explained by the fact that the sunlight has the ultraviolet component, which, striking the stone surface, deteriorates the stone color in the former case and improves it in the latter.