Very Strong fluorescence is the uppermost grade on the GIA fluorescence scale, assigned to diamonds that emit an exceptionally vivid — typically blue — glow when exposed to long-wave ultraviolet (UV) radiation at 365 nm. The reaction is so pronounced that it may be perceptible even under the UV component present in strong daylight or certain fluorescent lighting environments, distinguishing Very Strong stones from those graded merely Strong, Medium, Faint, or None. The grade is recorded on GIA Diamond Grading Reports and is routinely noted by other major laboratories including IGI and HRD.

The Physical Basis of Fluorescence

Fluorescence in diamonds arises primarily from structural defects and trace impurities within the crystal lattice. In the vast majority of fluorescent diamonds, the causative agent is nitrogen in specific aggregation states — particularly the N3 centre (three nitrogen atoms surrounding a vacancy), which absorbs UV radiation and re-emits it as visible blue light peaking at approximately 415–440 nm. Very Strong fluorescence indicates an unusually high concentration or particularly efficient arrangement of these centres. Less commonly, diamonds may exhibit yellow, orange, white, or red fluorescence, though blue remains overwhelmingly dominant across all fluorescence grades, including Very Strong.

Appearance Effects in Daylight

The practical significance of a Very Strong grade lies in its potential to alter a diamond's visible appearance under natural or mixed lighting. GIA research, published in Gems & Gemology, has documented that a subset of Very Strong fluorescent diamonds — estimated at roughly 35% of stones in that category in one study — display a hazy, oily, or milky transparency in face-up viewing under daylight-equivalent illumination. This phenomenon, sometimes called overblue or blue haze, results from the fluorescent emission partially obscuring the stone's inherent transparency and scintillation. The effect is not universal: many Very Strong fluorescent diamonds appear entirely unaffected to the eye, and a small proportion are judged to look more lively or whiter as a result of the blue emission counteracting a yellowish body colour.

The likelihood of visible haziness is not reliably predictable from the fluorescence grade alone; it depends on the specific distribution and nature of the fluorescent centres within the individual stone. Graders and buyers therefore benefit from direct visual assessment under standardised lighting conditions rather than relying solely on the report grade.

Market Pricing and Trade Conventions

Very Strong fluorescence consistently commands a price discount relative to non-fluorescent equivalents in the higher colour grades (D through H). The discount reflects both documented risk of haziness and longstanding market convention, which has historically treated strong fluorescence as a defect in near-colourless and colourless stones. In the D–F colour range, discounts for Very Strong fluorescence have been observed at 10–15% or more below comparable non-fluorescent stones in normal market conditions, though the precise differential fluctuates with supply, demand, and individual stone assessment.

Conversely, in lower colour grades — particularly I through M — the blue emission of Very Strong fluorescence can visually offset a warm body colour, making the stone appear whiter face-up than its graded colour would suggest. In these grades, the fluorescence discount narrows and may, in certain trade contexts, be absent or even reversed for stones that demonstrably benefit from the effect without exhibiting haziness.

Laboratory Reporting and Grading Methodology

GIA assesses fluorescence by exposing the diamond to a standardised long-wave UV source in a darkened environment and comparing the intensity of emission against a master set of reference stones. The five-grade scale — None, Faint, Medium, Strong, and Very Strong — is descriptive rather than quantitative; no specific lumen or spectrophotometric threshold defines the boundary between Strong and Very Strong. As a result, borderline stones may receive different grades from different laboratories or even different graders, and the grade should be understood as an approximation of intensity rather than a precise physical measurement.

Some laboratories, including Gübelin and SSEF, additionally comment on whether fluorescence has a perceptible effect on transparency or colour appearance, providing more actionable information than the grade alone.

Collector and Connoisseur Perspectives

A minority of buyers actively seek Very Strong fluorescent diamonds, particularly in lower colour grades where the visual benefit is tangible, or for the distinctive visual effect the stones produce under UV-rich environments such as discotheques or certain gallery lighting. Antique and vintage diamond jewellery frequently features strongly fluorescent stones, as earlier cutting and sourcing practices did not systematically discriminate against fluorescence; collectors of period pieces may therefore encounter Very Strong grades without the associated market discount applying in the same way as for modern commercial goods.