What Is Diamond Fluorescence? Good or Bad?

Short answer: Fluorescence is the faint glow, usually blue, that some diamonds give off under ultraviolet (UV) light. The report lists it as “None, Faint, Medium, Strong, Very Strong”. In daylight it does not affect how most stones look; in slightly yellowish stones, blue fluorescence can even make the stone look a little whiter. In very rare cases, very strong fluorescence can give a stone a slightly hazy look. Its effect on price is usually downwards.

What causes fluorescence?

Very small amounts of trace elements (such as nitrogen) and structural differences in a diamond’s crystal absorb ultraviolet light and give it back as visible light. So fluorescence is not a “flaw” but part of the stone’s identity. A large share of natural diamonds show fluorescence of varying strength; its most common colour is blue.

Fluorescence grades

You notice fluorescence most in sunlight and especially where there are UV lamps (such as the lighting in some venues): the stone gives off a faint blue glow.

Is fluorescence good or bad?

Sometimes an advantage

In stones with a slight warm tone, such as I or J, blue fluorescence can balance the yellowish tint and make the stone look a little whiter in daylight. Because these stones have fluorescence, they can also be priced lower; so it is possible both to look whiter and to pay less.

Sometimes a reason for care

In colourless stones (D–F), strong fluorescence lowers the price, because the market does not want fluorescence in these stones. Very strong fluorescence giving a stone a slightly hazy or oily look is rare but possible, so stones with “Very Strong” fluorescence should be chosen carefully.

How does fluorescence affect the price?

Generally, the stronger the fluorescence, the lower the price; the effect is greatest at the top colour grades. In the middle and lower colour grades the difference is smaller. This is an opportunity for those who want to make the most of their budget: a stone with “Medium” or “Strong” fluorescence that does not look hazy can be good value. Ways to make the most of your budget.

Fluorescence in lab-grown diamonds

Lab-grown diamonds usually have no fluorescence or only faint fluorescence; some stones may show weak fluorescence in colours different from natural diamonds (such as orangey or greenish). This difference is seen with laboratory instruments, not the eye, and it does not affect the stone’s look in daylight. Fluorescence is stated separately on the report for every stone. CVD and HPHT.

Frequently asked questions