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
Named Absorption Spectra – Foundation Tier
The twelve species below form the core absorption-spectrum vocabulary for the FGA Foundation
examination. Each spectrum must be recognisable by principal line(s) and overall pattern.
Note on wavelength values: Several nm values in this table are sourced from Read,
P. G. (ed.), Gemmology (commonly referenced as "Read 7th ed.", Butterworth-Heinemann), which
is the standard Gem-A teaching textbook. These values are [PARTIALLY_SUPPORTED]; the
chromophore assignments and general spectral patterns are confirmed by peer-reviewed
DOI-verified sources (Fritsch & Rossman 1987, DOI: 10.5741/gems.23.3.126; Dubinsky et al.
2020, DOI: 10.5741/gems.56.1.2; Breeding & Shigley 2009, DOI: 10.5741/gems.45.2.96;
Karampelas et al. 2019, DOI: 10.3390/min9090561; Balciūnaite et al. 2021,
DOI: 10.6001/chemija.v32i3-4.4549; Phillips & Talantsev 1996, DOI: 10.5741/gems.32.2.100),
but the precise nm values for some secondary lines derive from the textbook and are not
independently verifiable via abstract APIs.
| Species | Principal Lines (nm) | Chromophore | Diagnostic Strength |
|---|---|---|---|
| Ruby (natural + Verneuil) | 692.8 + 694.2 doublet; 668, 659; broad ~550 absorption; 468; UV cutoff ~400 | Cr³⁺ | Highest – doublet + luminescent glow in red light |
| Emerald (natural) | 683 + 680 doublet; 662, 646; broad ~580–630; blue-violet absorption; 477 (some stones) | Cr³⁺ (± Fe³⁺ in Fe-rich stones) | High – doublet diagnostic; Fe bands vary by origin |
| Blue sapphire | 450 strong narrow; 460, 470 weaker; broad Fe²⁺–Ti⁴⁺ CT absorption | Fe³⁺ + Fe²⁺–Ti⁴⁺ charge transfer | High – 450 nm line in virtually all blue sapphire |
| Almandine garnet | 504, 520, 573 broad bands; weaker 423, 461, 610, 680, 690 | Fe²⁺ | Highest – three-band pattern virtually diagnostic |
| Pyrope garnet | 504, 520, 573 (Fe²⁺); chromiferous: broad ~575 + 685–687 (Cr³⁺) | Fe²⁺ ± Cr³⁺ | Moderate – Fe bands shared with almandine |
| Spessartine garnet | 410, 421, 432 strong (Mn²⁺); violet edge cutoff | Mn²⁺ | Highest – Mn triplet unique to spessartine |
| Demantoid garnet | ~440 nm cutoff (Fe³⁺); Russian type: 618, 634, 685, 701 (Cr³⁺) | Fe³⁺ ± Cr³⁺ | High – 440 cutoff separates from green idocrase |
| Peridot | 493, 473, 453 three evenly spaced bands (Fe²⁺) | Fe²⁺ | Highest – three-iron-band pattern essentially diagnostic |
| Zircon (high / Sri Lanka type) | 653.5 principal; 691, 662, 660, 621, 615, 589, 562, 537, 516, 484, 460 fine lines | U⁴⁺ | Highest – picket-fence multi-line pattern unmistakeable |
| Diamond (Cape series / Type Ia) | 415.5 (N3 centre); 478 (N2); 465, 451, 435, 423, 401 weaker vibronic series | Nitrogen aggregate (N3, N2 defect centres) | High – 415.5 nm canonical Cape yellow marker |
| Red spinel (natural) | 656, 665, 685 narrow Cr³⁺ lines; broad ~540 absorption | Cr³⁺ | High – organ pipe triplet separates from ruby (694 nm doublet) |
| Zircon (low / metamict) | 653.5 diffuse or absent; featureless or very broad | U⁴⁺ (amorphised lattice) | Low – contrast with high zircon confirms metamict character |
Named Absorption Spectra – Diploma Extension Tier
The sixteen species below extend the Foundation spectra list to Diploma level, covering
additional corundum varieties, tourmalines, secondary garnets, and selected rare species.
The same note on textbook-sourced nm values applies (see Foundation tier heading above).
| Species | Principal Lines (nm) | Chromophore | Diagnostic Strength |
|---|---|---|---|
| Alexandrite (chrysoberyl) | 680.5 + 678.5 doublet; 655, 645; broad ~580 (colour-change band); broad ~420–450 | Cr³⁺ | High – doublet shifted from ruby (694 nm) and emerald (683 nm) |
| Yellow sapphire | 450 (Fe³⁺, if present); broad tailing into violet; often absent in pale stones | Fe³⁺ ± colour centre | Low–moderate – spectrum often very weak; RI/SG primary |
| Green sapphire | 471, 460, 450 (Fe²⁺/Fe³⁺) | Fe²⁺ + Fe³⁺ | Moderate – modified blue sapphire pattern |
| Padparadscha sapphire | 450 (Fe³⁺); broad violet colour-centre absorption | Fe³⁺ + colour centre | Low – combination not uniquely diagnostic; colour description primary |
| Colombian vs Zambian emerald | Colombia: Cr doublet 683/680, Fe bands weak/absent; Zambia: same doublet + stronger Fe absorption + possible 477 nm line | Cr³⁺ ± Fe³⁺ | Moderate – Fe-band presence/absence aids origin discrimination; not conclusive alone |
| Tsavorite garnet (chrome grossular) | Broad ~630 (V³⁺/Cr³⁺); broad ~450; weak 433 (Mn²⁺ if present) | V³⁺ ± Cr³⁺ ± Mn²⁺ | Moderate – no single sharp diagnostic line; RI/SG more reliable |
| Rhodolite garnet (pyrope-almandine) | 504, 520, 573 (Fe²⁺); Cr³⁺ shoulder ~680–687 if chromiferous | Fe²⁺ ± Cr³⁺ | Moderate – identical to almandine/pyrope; SG/RI differentiate |
| Indicolite tourmaline | Broad ~720 (Fe²⁺ d-d); broad UV cutoff 300–400 (O²⁻–Fe³⁺ CT) | Fe²⁺ + Fe²⁺–Fe³⁺ IVCT | Moderate – gradual cutoff, no sharp lines; contrast with blue sapphire 450 nm line |
| Rubellite tourmaline | Broad ~520 (Mn³⁺); no sharp lines | Mn³⁺ | Moderate – broad green absorption; no lines (contrast with red spinel organ pipe) |
| Chrome tourmaline | ~680 (Cr³⁺); broad green/blue absorption | Cr³⁺ | Moderate–high – Cr line position less sharp than ruby or emerald |
| Aquamarine (blue beryl) | 537, 456, 427 (Fe²⁺ + Fe³⁺); often weak | Fe²⁺ + Fe³⁺ | Low–moderate – spectrum frequently faint in pale stones |
| Tanzanite (heat-treated blue zoisite) | Broad ~450–460 (V³⁺ along α-axis); broad ~520 (V³⁺) | V³⁺ ± Ti³⁺/⁴⁺ | Moderate – broad V band; trichroism is stronger diagnostic indicator |
| Jadeite | 437 narrow (Fe³⁺, most jadeite); imperial green: broad ~630–650 + ~690 (Cr³⁺) | Fe³⁺ ± Cr³⁺ | Moderate – 437 nm separates from nephrite (which lacks this line) |
| Chrome diopside | Broad ~670 (Cr³⁺); ~690 narrow-moderate Cr R-lines | Cr³⁺ | Moderate – broad 670 band similar to but distinguishable from ruby and demantoid |
| Sphene (titanite) | ~580–585 doublet (Nd³⁺ + Pr³⁺ REE); additional sharp REE lines | Nd³⁺, Pr³⁺ (rare earth) | Moderate – didymium REE pattern + extraordinary dispersion makes misidentification unlikely |
| Synthetic emerald (flux, e.g. Chatham) vs natural | Cr doublet 683/680 present; Fe-related bands (477 nm, blue-violet) absent or very weak | Cr³⁺ (Fe absent) | Moderate – Fe-band absence supports synthetic; also consistent with Fe-poor Colombian natural; microscopy required for confirmation |