Trace elements caught in the crystal as it formed cause some diamonds to emit visible light under ultraviolet. Roughly a third of diamonds show it to some degree. In the large majority of those the glow is blue, though yellow, orange and white occur.

The grades

• None: no reaction under ultraviolet.
• Faint: barely detectable, and of no practical consequence.
• Medium: visible under a UV lamp, essentially invisible in daily wear.
• Strong and Very Strong: an obvious glow under UV, and the only grades that can affect how the stone looks in daylight.

Daylight contains ultraviolet, so a strongly fluorescent stone will react outdoors as well as under a lamp. Indoors, under ordinary lighting, there is nothing to see.

When it works in your favour

Blue sits opposite yellow. In a stone carrying a faint warm tint, typically the I to K grades, medium or strong blue fluorescence can cancel some of that yellow and make the diamond face up whiter than its grade suggests.

Since fluorescent stones usually sell at a discount, this is one of the few places in the trade where the cheaper option can be the better looking one.

When it works against you

In the top colour grades, D through F, the market marks fluorescent stones down. Buyers paying for absolute colourlessness want nothing else going on in the crystal, and that preference is priced in whether or not you can see a difference.

Separately, in a small number of Very Strong cases, the stone can look faintly hazy, milky or oily in daylight. This is uncommon, but it is real, and it is the origin of the whole reputation.

What we would tell you across the counter

Judge the stone, not the line on the report. Fluorescence is the one characteristic where the certificate can mislead you in both directions: it can warn you off a diamond that looks superb, and it can say nothing about one that looks hazy.

Ask to see any fluorescent stone in daylight before you decide. If it looks clean and bright out there, the discount is yours to take.