Longwave UV (365 nm)

Longwave UV is the ultraviolet wavelength range from roughly 315 nm to 400 nm. GIA grades diamond fluorescence using a standard longwave UV lamp at 365 nm, which is the wavelength that most reliably triggers the N3-center blue fluorescence response in natural Type Ia diamond. The blacklight in a jeweller's booth or display case is almost always longwave UV; sunlight contains longwave UV plus shortwave UV (200 nm to 280 nm, blocked by the atmosphere) and midwave UV (280 nm to 315 nm, partially blocked) and the full visible spectrum.

The grading wavelength matters because diamond fluoresces differently under different UV bands. Natural Type Ia diamonds glow blue under 365 nm longwave UV. Some treated and lab-grown diamonds glow differently — for example, CVD lab-grown stones often glow orange or red under DiamondView (a shortwave UV imaging system), which is how GIA and IGI separate natural from CVD origin. The wavelength specificity is what makes fluorescence behavior useful as an identification signal as well as a grading metric.

Daylight contains longwave UV. The amount varies with time of day, season, latitude, and weather — overhead summer noon has the most, late afternoon and winter days less. A stone graded Strong Blue under GIA longwave will glow visibly in direct outdoor daylight but more weakly indoors near a window. The milky-fluorescence risk that the trade discounts on Strong Blue D-G stones is a daylight-exposure risk, not an everywhere-always risk; a stone worn primarily indoors under tungsten or LED lighting (neither of which produces material UV) will not show milkiness.

Standard household lighting produces no longwave UV. Incandescent bulbs are pure black-body radiators below the UV band. LED lighting is engineered to peak in the visible spectrum and has negligible UV output. Fluorescent tubes produce some UV but the phosphor coating absorbs almost all of it. The result is that a fluorescent diamond worn under typical home or office lighting will not visibly fluoresce — the fluorescence behavior only emerges in daylight, jeweller blacklights, and outdoor conditions.

The behavior is reversible and instantaneous. A diamond placed under longwave UV glows immediately; remove the UV and the glow stops. There is no afterglow or phosphorescence in normal diamond. Some treated diamonds and certain rare type IIa naturals show phosphorescence — a brief afterglow after UV is removed — which is one of the routes a lab uses to flag a stone for further testing.