Introduction
Spectroscopy is the definitive tool in lab-grown diamond identification. Where UV lamps, polarisers, and microscopes provide screening indicators — clues that raise or lower suspicion — spectroscopic instruments probe the diamond's atomic structure directly. They detect specific defect centres in the crystal lattice that are characteristic of how the diamond was formed and what happened to it afterward.
Three spectroscopic techniques dominate diamond identification: FTIR (Fourier Transform Infrared), photoluminescence (PL), and UV-Vis absorption. Each measures something different about the diamond's internal chemistry, and together they build a detailed profile that can distinguish natural from lab-grown — and CVD from HPHT — with high confidence.
FTIR: Fourier Transform Infrared Spectroscopy
What It Measures
FTIR spectroscopy measures how a diamond absorbs infrared light at different wavelengths. The absorption pattern reveals the presence and configuration of impurity atoms — primarily nitrogen — within the crystal lattice.
Why It Matters for Screening
Nitrogen configuration is the single most effective screening filter in diamond identification:
Type Ia (A and B aggregates): Nitrogen atoms have aggregated into pairs (A centres) or clusters of four (B centres) over billions of years. This aggregation requires geological time — it does not happen during the short growth periods of laboratory synthesis. Type Ia accounts for 95–98 % of natural diamonds.
Type Ib (isolated nitrogen): Nitrogen atoms exist as isolated substitutional atoms. This is the initial state of nitrogen in diamond before aggregation begins. Type Ib is rare in nature (less than 0.1 % of natural diamonds) but common in HPHT-grown diamonds produced in nitrogen-containing atmospheres.
Type IIa (no measurable nitrogen): The purest diamond type. Only 1–2 % of natural diamonds are Type IIa, but most gem-quality CVD diamonds (grown in nitrogen-free environments) and colourless HPHT diamonds fall into this category.
Type IIb (boron instead of nitrogen): Contains boron as the dominant impurity. Extremely rare in nature but produced deliberately in HPHT growth by adding boron to the flux.
A Type Ia result on FTIR effectively clears the diamond as natural. A Type II result triggers advanced testing.
FTIR Absorption Peaks
Key absorption features include:
- 1282 cm⁻¹ (A aggregate): Nitrogen pairs — characteristic of Type Ia
- 1175 cm⁻¹ (B aggregate): Nitrogen clusters — also Type Ia
- 1130 cm⁻¹ (isolated nitrogen): Single nitrogen atoms — Type Ib
- 2800 cm⁻¹ region (boron): Boron absorption — Type IIb
Photoluminescence Spectroscopy
What It Measures
Photoluminescence (PL) spectroscopy illuminates the diamond with a laser and measures the wavelengths of light emitted as the crystal's defect centres relax from excited states. Each defect centre produces a characteristic emission peak at a specific wavelength — a spectroscopic fingerprint.
Growth-Method-Specific Signatures
PL spectroscopy is the most powerful tool for determining which growth method produced a lab-grown diamond:
CVD diagnostic — SiV⁻ at 736.6/736.9 nm: The silicon vacancy centre is caused by silicon atoms from the CVD chamber walls or seed holder becoming incorporated into the growing crystal. It produces a characteristic doublet in PL spectra. This defect is essentially absent in natural diamonds and HPHT-grown diamonds, making it one of the most reliable CVD markers.
HPHT diagnostic — Nickel defects at 882/884 nm: Nickel from the metal flux catalyst can enter the diamond lattice, creating specific defect centres that emit at 882 and 884 nm. These are characteristic of HPHT growth and are not found in natural diamonds or CVD material.
Natural diagnostic — N3 centre at 415.2 nm: The N3 centre consists of three nitrogen atoms surrounding a vacancy. It forms only when nitrogen has had sufficient time to aggregate — a process requiring geological timescales. The presence of N3 is strong evidence of natural origin.
Additional PL Features
- H3 (503.2 nm): A nitrogen-vacancy-nitrogen centre found in both natural and treated diamonds. Its presence alone is not diagnostic, but its context within the full PL spectrum provides additional information.
- NV⁻ (637 nm): A nitrogen-vacancy centre. Found in various diamond types but can indicate treatment when combined with other features.
- GR1 (741 nm): A neutral vacancy centre associated with irradiation damage.
UV-Vis Absorption Spectroscopy
What It Measures
UV-Vis absorption spectroscopy measures how much light a diamond absorbs at each wavelength across the ultraviolet and visible spectrum. The absorption pattern reveals which defect centres are present and contributing to the diamond's colour.
Diagnostic Value
UV-Vis spectroscopy is particularly useful for:
- Confirming natural colour: The N3 absorption at 415.2 nm, combined with the cape series (N2 at 478 nm and related features), confirms that yellow colour in a diamond results from natural nitrogen aggregation.
- Detecting treatment: Post-growth HPHT treatment modifies specific absorption features. A CVD diamond that has been HPHT-treated shows a different absorption profile than untreated material.
- Colour origin determination: Distinguishing whether a diamond's colour is natural, as-grown, or the result of treatment.
How the Three Techniques Work Together
| Technique | Primary Question | Key Indicators |
|---|---|---|
| FTIR | What type is this diamond? | Nitrogen configuration (Ia, Ib, IIa, IIb) |
| PL | What growth method produced it? | SiV⁻ (CVD), Ni (HPHT), N3 (natural) |
| UV-Vis | What causes its colour? | Cape series (natural), treatment signatures |
In practice, the workflow is:
- FTIR screens by type — Type Ia passes as natural, Type II refers
- PL identifies growth-method-specific defect centres — SiV⁻ confirms CVD, Ni confirms HPHT, N3 supports natural
- UV-Vis clarifies colour origin and treatment history
Equipment and Access
Spectroscopic instruments are laboratory-grade equipment — not bench tools for retail jewellers. FTIR and PL spectrometers are standard in major gemological laboratories (GIA, HRD, IGI) and specialised diamond testing centres. Their use requires trained operators and controlled conditions (PL spectroscopy, for example, is often performed at cryogenic temperatures to sharpen spectral features).
For retail and trade settings, automated instruments like the GIA iD100 incorporate simplified spectroscopic analysis into their screening algorithms, providing spectroscopy-informed results without requiring the operator to interpret raw spectra.
Frequently Asked Questions
Can spectroscopy definitively identify a lab-grown diamond?
In most cases, yes. The combination of FTIR type determination and PL defect centre identification provides definitive origin determination for the vast majority of diamonds. Ambiguous cases are rare and typically involve unusual natural Type IIa diamonds.
What is the SiV⁻ defect and why does it matter?
The silicon vacancy minus centre (SiV⁻) is a lattice defect where a silicon atom occupies a position between two vacant carbon sites. It emits at 736.6/736.9 nm in photoluminescence. It occurs in CVD diamonds due to silicon contamination from the growth chamber and is essentially absent in natural and HPHT diamonds — making it a definitive CVD marker.
Do natural diamonds ever show nickel defects?
Nickel-related defects can occur in some natural diamonds from certain geological environments, but the specific combination of features and their relative intensities in HPHT diamonds differ from natural occurrences. Experienced spectroscopists can distinguish the two contexts.
Is spectroscopy destructive?
No. All three techniques are non-destructive. The diamond is illuminated with infrared light (FTIR), laser light (PL), or UV/visible light (UV-Vis) and observed — nothing is altered, removed, or damaged.
Summary
Spectroscopy provides the most definitive tools for lab-grown diamond identification. FTIR determines diamond type through nitrogen configuration, immediately clearing Type Ia stones as natural. Photoluminescence detects growth-method-specific defect centres — SiV⁻ for CVD, nickel for HPHT, N3 for natural — providing a molecular-level fingerprint of origin. UV-Vis absorption clarifies colour causation and treatment history. Together, these three techniques establish origin with high confidence and form the backbone of every major gemological laboratory's identification protocol.