Ultraviolet examination

Ultraviolet light is invisible to the human eye. When it strikes certain gemstones, they can emit visible light: they fluoresce. Ultraviolet examination makes such reactions visible and is part of the standard repertoire of gemmological laboratories.

Long-wave and short-wave

Two ranges of wavelength are used: long-wave UV around 365 nanometres and short-wave UV around 254 nanometres. Many materials react differently to each. The stone is viewed in a darkened cabinet, and the colour, strength and distribution of the glow are recorded.

Fluorescence and phosphorescence

Fluorescence is light a stone emits while it is being excited. If it continues to glow after the lamp is switched off, this is called phosphorescence. Both can provide clues.

Typical observations

• Ruby: often a distinct red under long-wave UV, caused by chromium; iron-rich rubies react more weakly
• Diamond: frequently blue fluorescence of varying strength; recorded as a separate entry in diamond reports
• Synthetic stones: sometimes characteristic patterns, for example in some synthetic diamonds under short-wave UV
• Fillings: some oils and resins in emeralds fluoresce and reveal filled fractures

The limits

Fluorescence is supplementary information. The same material can glow strongly or not at all depending on its trace elements. The absence of a reaction proves nothing, and neither does its presence on its own. Modern laboratories also use imaging techniques that reveal growth patterns under UV, for example to separate natural from synthetic diamonds.

Safety

Short-wave UV can damage eyes and skin. The examination is therefore carried out in closed cabinets or with protective glasses.

Fluorescence in everyday life

In diamonds, fluorescence can sometimes affect appearance: very strong blue fluorescence may make a stone look slightly hazy in sunlight, while faint fluorescence usually has no visible effect.

In brief

The UV reaction is a supplementary observation. It can point to fillings, synthetics or particular elements, but on its own proves nothing. In diamonds, fluorescence is a separate entry in the report and only affects appearance when it is strong.

At NOBILITH

If a laboratory report states fluorescence, it appears in the stone's dossier. We consider it together with the other information, not as a separate verdict on quality.

Sources

• GIA: Gem Encyclopedia, fluorescence
• Nassau, K.: The Physics and Chemistry of Color