The Spectroscope
Using the spectroscope to observe absorption spectra, identifying characteristic patterns for gem species and detecting treatments.
Introduction
The spectroscope displays the absorption spectrum of a gem, showing which wavelengths
of light are absorbed. Certain absorption patterns are diagnostic for specific gems
and treatments, making this an essential identification tool.
While requiring more skill than basic instruments, the spectroscope provides
information unavailable from any other portable tool.
Types of Spectroscope
Two main types are used in gemmology:
Prism Spectroscope
Uses a prism to disperse light into its spectrum:
- Better resolution in red: Red end of spectrum spread more
- Compressed blue end: Blue/violet more difficult to read
- Traditional choice: Many gemmologists prefer prism type
- Wavelength scale: May be non-linear due to prism dispersion
Diffraction Grating Spectroscope
Uses a diffraction grating to create the spectrum:
- Even spread: Spectrum evenly distributed
- Easier wavelength estimation: More linear scale
- Good for learning: Easier to correlate with reference charts
- May have multiple orders: Secondary spectra visible
Setup and Technique
Proper setup is critical for useful observations:
Light Source Requirements
- Strong white light (incandescent preferred)
- Light transmitted through the gem
- Position slit close to the stone
- Avoid ambient light interference
Viewing Procedure
- Illuminate the gem with strong transmitted light
- Position the spectroscope slit close to the stone
- Focus the eyepiece on the spectrum
- Allow eyes to adapt (30 seconds to 1 minute)
- Look for dark lines or bands against the coloured spectrum
- Note position (wavelength in nm) and width of features
Recording Observations
Document findings systematically:
- Lines: Sharp, narrow features (atomic absorption)
- Bands: Broader absorption regions
- Cut-off: Absorption of entire spectral region (e.g., blue cut-off)
- Position: Wavelength in nanometres (nm)
- Intensity: Weak, moderate, strong
Diagnostic Absorption Spectra
| Gemstone | Key Features | Wavelength (nm) |
|---|---|---|
| Ruby/red spinel | Cr doublet in red | 694 (close doublet) |
| Ruby | Additional Cr lines + fluorescent glow | 668, 659, plus organ pipe lines |
| Emerald (natural) | Cr line + Fe bands | 683 + bands in red/yellow |
| Blue sapphire | Fe bands | 450, 460, 470 (three-band) |
| Almandine garnet | Fe bands | 505, 520, 575 (diagnostic trio) |
| Zircon (high) | Uranium lines | Many fine lines throughout spectrum |
| Demantoid garnet | Cr line | 443 (horse line; diagnostic) |
| Peridot | Fe bands | 493, 473, 453 (three evenly spaced) |
| Yellow sapphire | Fe bands | 450 band region |
| Yellow apatite | Rare earth lines | Multiple fine lines in yellow-green |
Chromophore Absorption Patterns
Different colour-causing elements produce characteristic spectra:
Chromium (Cr³⁺)
Produces red colour with characteristic features:
- Doublet at 694nm: Very close pair of lines in deep red
- Lines at 668, 659nm: Additional absorption in red
- Broad absorption in yellow-green: Creates red colour
- Found in: Ruby, emerald, red spinel, alexandrite, chrome tourmaline
Iron (Fe)
Most common chromophore in gems:
- Fe²⁺: Produces blue-green colours; broad bands
- Fe³⁺: Produces yellow-brown colours; sharp lines
- Fe²⁺-Fe³⁺ charge transfer: Intense blue (sapphire)
- Found in: Sapphire, peridot, aquamarine, almandine, most gems
Copper (Cu)
Unusual chromophore in gems:
- Cu²⁺: Broad absorption in red region
- Creates blue-green to blue colours
- Found in: Turquoise, Paraíba tourmaline, malachite
Rare Earth Elements
Produce sharp, narrow absorption lines:
- Didymium (Nd + Pr): Yellow-green lines
- Found in: Apatite, sphene, some zircons
- Useful for distinguishing species
Spectrum Reading Tips
Treatment Detection
The spectroscope can reveal certain treatments:
Heat Treatment Signs
- Modified absorption patterns in some treated stones
- Reduced Fe bands in heated sapphires
- Changed colour centre absorption
Diffusion Treatment
- Surface-related absorption (best seen immersed)
- Unusual colour distribution
Dyed Materials
- Atypical absorption patterns
- Absorption inconsistent with body colour
- Unusual dye bands
Limitations
Be aware of spectroscope limitations:
- Requires adequate light transmission
- Very pale stones show weak spectra
- Small stones are difficult to examine
- Takes practice to interpret patterns
- Some gem species show no diagnostic spectrum
- Laboratory spectrometers provide more detailed analysis