In most cases, fluorescence does not affect the diamond's appearance in ordinary indoor light. Sometimes, a Medium blue fluorescence can actually make a yellowish diamond (J–K color) look whiter in sunlight, which is advantageous. But Strong fluorescence ('Strong' or 'Very Strong') can in rare cases produce a milky, hazy surface that impairs transparency and brilliance. GIA research shows this affects a minority of stones with strong fluorescence.

Fluorescence is more common than many realize:

• 25–35% of all natural diamonds exhibit some degree of fluorescence.

• Of these, the majority are blue (~97%) – blue fluorescence is caused by nitrogen impurities in the crystal structure.

• Only 3–5% of all diamonds have Strong or Very Strong fluorescence.

• GIA's grading scale: None, Faint, Medium, Strong, Very Strong.

GIA's longitudinal studies confirm that fluorescence in the vast majority of cases does not affect the visual impression in normal daylight.

Blue is by far the most common fluorescence color, but diamonds can exhibit several shades:

• Blue (most common): Caused by nitrogen in the crystal lattice. Can neutralize yellowish tones in J–K diamonds.

• Yellow: Seen in diamonds with a high concentration of a specific type of nitrogen. Can enhance an existing yellowish tint.

• White/green: Uncommon, occurs in stones from certain South African mines.

• Orange: Very rare, sometimes associated with naturally orange-colored 'fancy' diamonds.

Price effect: A D–F (colorless) diamond with Strong fluorescence typically trades at a small discount. A J–K diamond with Medium blue fluorescence can actually be priced *higher* by some buyers thanks to its visually whiter appearance in sunlight.