The UV Lamp
Using ultraviolet light for fluorescence testing, distinguishing LWUV from SWUV reactions, and safety considerations.
Introduction
Ultraviolet fluorescence can be diagnostic for certain gems and treatments.
Two wavelengths are used: long-wave (LWUV, 365nm) and short-wave (SWUV, 254nm).
Different gems may react differently to each wavelength.
While rarely definitive alone, UV testing provides valuable supporting evidence
for identification.
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
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
- Place gem on white paper in darkness
- Observe under LWUV first (safer)
- Note colour, intensity, and distribution
- Switch to SWUV (use eye protection)
- Note any differences from LWUV
- Check for phosphorescence after UV removed
- Document all observations
Describing Fluorescence
Record fluorescence systematically:
- 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
| 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
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
- Unusual colours may indicate treatment or synthetic
- CVD diamonds often show orange fluorescence
- 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