Diamond fluorescence

Fluorescence is a diamond’s visible reaction when exposed to ultraviolet light. It is not one of the 4Cs and should not be treated as automatically good or bad.

How it is described

GIA describes fluorescence strength as None, Faint, Medium, Strong or Very Strong. For Medium, Strong and Very Strong fluorescence, the colour is also stated. Blue is common, but other colours occur.

How it may affect appearance

The effect depends on the stone, strength and lighting. Fluorescence can be difficult to notice in ordinary indoor light. Under UV-rich light, such as strong sunlight, the reaction may become clearer. In rare cases, diamonds with Very Strong fluorescence may appear hazy or oily; assess the actual stone.

Fluorescence and colour grade

In some lighting, blue fluorescence can affect how a diamond with a yellow tint appears because the colours sit on opposite sides of the colour spectrum. It does not change the graded D–Z colour stated in the report.

How to compare

• Read the strength and colour in the grading report.
• View the diamond in daylight and ordinary indoor light.
• Compare it with a similar stone that has no fluorescence.
• Assess the actual stone instead of rejecting a report detail in advance.

Fluorescence may affect price and preference, but it does not by itself describe the diamond’s overall quality.

Read about colour, grading reports or compare diamonds.