The UV Lamp

Using ultraviolet light for fluorescence testing, distinguishing LWUV from SWUV reactions, and safety considerations.

By Fabian Moor Last updated
UV fluorescence phosphorescence identification

Introduction

The UV lamp excites fluorescence in gemstones by irradiating them with ultraviolet light
at two standard wavelengths: long-wave (LWUV, 365 nm) and short-wave (SWUV, 254 nm).
Fluorescence is the immediate emission of visible light while UV is applied; phosphorescence
is the afterglow that persists after the source is removed.

Testing with both wavelengths is important because some gems react differently to each:
most natural diamonds fluoresce blue under LWUV but show little or no reaction to SWUV,
whereas CVD synthetic diamonds often display orange fluorescence under LWUV, a useful
screening indicator. [1] Diamond type also affects fluorescence:
Type IIa diamonds (nitrogen-free) often show unusual or inert LWUV reactions, prompting
referral for FTIR or PL testing. [2] Emeralds provide a
useful contrast: natural emeralds are usually inert under both wavelengths, while many
synthetic emeralds fluoresce red. UV testing is rapid, non-contact, and non-destructive,
but results vary within species and should always be correlated with RI, spectroscope, and
microscope data. [3]

LWUV vs SWUV

Understanding the two wavelengths:

Long-Wave UV (LWUV, 365nm)

  • Safer: Less harmful to eyes
  • Penetrates deeper: May show internal reactions
  • Often stronger reactions: Many gems fluoresce more to LWUV
  • Common light sources: LED-based lamps, Wood's lamps [3]

Short-Wave UV (SWUV, 254nm)

  • More diagnostic: Different reactions from LWUV
  • Surface reactions: Less penetration
  • Requires eye protection: Can damage eyes
  • Requires special tubes: Mercury vapour lamps

Why Use Both

Testing with both wavelengths is important because:

  • Some gems fluoresce only to one wavelength
  • Different colours may appear under each
  • The pattern of LWUV vs SWUV reaction can be diagnostic
  • Treatments may show different reactions to each

Testing Procedure

For accurate fluorescence observation:

Setup Requirements

  • Complete darkness: Essential for accurate observation
  • UV cabinet or darkroom: Blocks ambient light
  • Clean gems: Oils and dirt can fluoresce
  • White background: For consistent observation

Observation Protocol

  1. Place gem on white paper in darkness
  2. Observe under LWUV first (safer)
  3. Note colour, intensity, and distribution
  4. Switch to SWUV (use eye protection)
  5. Note any differences from LWUV
  6. Check for phosphorescence after UV removed
  7. Document all observations

Describing Fluorescence

Record fluorescence systematically: [4]

  • Colour: Blue, red, yellow, green, orange, white
  • Intensity: Inert, weak, moderate, strong, very strong
  • Distribution: Even, patchy, zoned
  • LWUV vs SWUV: Note differences

Safety Considerations

Diagnostic Fluorescence Reactions

UV Fluorescence by Gemstone
Gemstone LWUV SWUV Notes
Diamond (most) Blue Blue (may differ) Strong blue common; some inert
Ruby (natural) Strong red Moderate red Due to Cr content
Ruby (synthetic) Very strong red Strong red Often stronger than natural
Emerald (natural) Usually inert Inert Occasionally weak red
Emerald (synthetic) Often red Red Hydrothermal may show less
Natural pearl Usually inert Inert or weak Variable
Cultured pearl Often bluish Variable May show different zones
Kunzite Orange/pink Orange/pink Also phosphorescent
Amber (Baltic) Blue-white Variable Blue fluorescence typical
Opal Variable Variable White to green common
Synthetic spinel (colourless) Blue-white Blue-white Used as diamond simulant

Phosphorescence

Some gems continue glowing after UV light is removed:

What Is Phosphorescence?

Phosphorescence is delayed fluorescence – the gem continues to emit light
for seconds to minutes after the UV source is removed.

This "afterglow" can be diagnostic for certain materials.

Phosphorescent Gems

Gem Phosphorescence Duration
Kunzite Orange-pink Several seconds to minutes
Diamond (some) Variable colours Seconds
Synthetic spinel Blue-white Brief
Calcite Pink or white Variable
Some pearls Weak Brief

Fluorescence in Identification

Practical applications of UV testing:

Natural vs Synthetic

  • Synthetic ruby: Usually stronger fluorescence than natural
  • Synthetic emerald: May fluoresce red (natural usually inert)
  • Synthetic spinel: Strong blue-white fluorescence

Treatment Detection

  • Glass filling: May fluoresce differently from host
  • Polymer treatment: Some polymers fluoresce
  • Coating detection: Coating may have different reaction

Diamond Screening

Fluorescence helps with diamond identification:

  • Strong blue fluorescence suggests natural Type Ia [2]
  • Unusual colours may indicate treatment or synthetic
  • CVD diamonds often show orange fluorescence [1]
  • HPHT-treated may show different patterns

Limitations

UV testing has important limitations:

  • Not definitive: Similar gems may show similar reactions
  • Variable within species: Not all specimens react the same
  • Affected by treatments: Some treatments alter fluorescence
  • Dirty surfaces: Can give false readings
  • Coloured stones: Body colour may mask weak fluorescence

References

  1. ↑ 1. Willems, B.; Tallaire, A.; Barjon, J. (2011). Exploring the Origin and Nature of Luminescent Regions in CVD Synthetic Diamond. Gems & Gemology, 47(3), 202–207. DOI: 10.5741/gems.47.3.202.
  2. ↑ 2. Breeding, C.; Shigley, J. (2009). The 'Type' Classification System of Diamonds and Its Importance in Gemology. Gems & Gemology, 45(2), 96–111. DOI: 10.5741/gems.45.2.96.
  3. ↑ 3. Read, P. (2014). Gemmology (3 ed.). Butterworth-Heinemann. ISBN: 978-0-08-050722-4. DOI: 10.4324/9780080507224.
  4. ↑ 4. Anderson, B. (1990). Gem Testing (10 ed.). Butterworth-Heinemann.

Related Topics

Optical PropertiesSynthetic Gemstones