Diamond Screening Instruments
Thermal conductivity probes, electrical conductivity testers, DiamondView, DiamondSure, and reflectance meters – the tools used to screen loose diamonds and detect synthetic and simulant materials.
Introduction
Diamonds require dedicated screening instruments because their refractive index (2.417) and
specific gravity (3.52) exceed the range of the standard gemmological refractometer and are
therefore not directly confirmable by the standard tool set. The instruments on this page
address that gap – from rapid in-trade thermal probes through to laboratory-grade UV imaging
systems used to detect synthetic growth patterns.
Thermal Conductivity Probe
Principle: Diamond has the highest thermal conductivity of any natural substance –
approximately 2000–2200 W·m⁻¹·K⁻¹, compared with ~100 W·m⁻¹·K⁻¹ for most gem minerals.
A diamond tester consists of a heated tip; when pressed against the stone surface, the rate of
heat dissipation is measured electronically. Diamond dissipates heat so rapidly that the meter
deflects into the "diamond" zone, while glass, cubic zirconia (CZ), and most gem minerals
dissipate heat slowly and read below the threshold.
Procedure
- Allow the probe tip to warm to operating temperature – the LED ready indicator on
the unit confirms when the tip is at calibrated temperature. - Hold the stone firmly on a clean, flat surface.
- Touch the probe tip perpendicularly to a polished facet with gentle, consistent
pressure; do not allow the stone to rock. - Read the result: "diamond" (deflects into upper zone) or "not diamond" (lower zone).
- For any "diamond" reading: confirm with an electrical conductivity test (see below)
and, if still uncertain, with a spectroscope or polariscope (moissanite shows strong
birefringence; diamond does not).
Thermal Test Fails for Moissanite
Moissanite (synthetic silicon carbide, SiC) has a thermal conductivity of
approximately 490 W·m⁻¹·K⁻¹ – lower than diamond but high enough to trigger the
"diamond" reading on most standard single-probe thermal testers.
Henn (2021) reported that synthetic moissanite testing as "diamond" using standard
diamond testers is a well-documented problem: Journal of Gemmology 37(8), 778–779
(DOI: 10.15506/jog.2021.37.8.778) [VERIFIED].
Speich et al. (2022) reported a synthetic moissanite with diamond-level reflectivity that
also defeated thermal testers, reinforcing the need for the dual electrical test:
Journal of Gemmology 38(4), 323–325 (DOI: 10.15506/jog.2022.38.4.323) [VERIFIED].
Electrical Conductivity Overlay
Why electrical testing is needed: Type IIb natural diamond (containing boron impurities)
is a p-type semiconductor with measurable electrical conductivity. Most natural diamonds
(Type Ia, Ib, IIa) are electrical insulators. Moissanite is a semiconductor – it conducts
electricity at gem-testing voltages.
Dedicated moissanite testers (combining thermal and electrical measurement) exploit this
difference:
| Result | Thermal | Electrical | Conclusion |
|---|---|---|---|
| Diamond (Type Ia/Ib/IIa) | High (diamond zone) | Insulator | Diamond |
| Diamond (Type IIb – blue/grey) | High | Conductor | Natural Type IIb diamond – do not reject as moissanite |
| Moissanite | High (passes thermal) | Conductor | Moissanite |
| CZ and most simulants | Low (fails thermal) | Insulator | Non-diamond simulant |
The combined thermal + electrical test discriminates all three major categories. When
testing a blue or grey diamond suspected of being Type IIb: test thermally first, then
electrically – Type IIb will pass thermal and pass electrical, while moissanite also passes
both. Further confirmation (spectroscope: moissanite shows strong birefringence; diamond
does not) is required for blue/grey stones.
DiamondView (Deep-UV Fluorescence Imaging)
Principle: DiamondView (a De Beers/Element Six instrument) irradiates the diamond surface
with very short-wavelength UV light below 230 nm – beyond the range of standard SW-UV lamps
(~254 nm). This deep UV excites strong surface fluorescence in diamond, revealing growth
sector patterns.
Fluorescence Pattern Interpretation
Natural diamond, HPHT synthetic diamond, and CVD synthetic diamond show distinctly
different fluorescence patterns because their growth sectors and defect distributions
differ:
- Natural diamond: typically blue or blue-green fluorescence; irregular,
non-geometric patterns without sharp sector boundaries. - HPHT synthetic diamond: characteristic cuboctahedral growth sector pattern –
distinct coloured sectors (blue, yellow-green, orange) in a geometrically regular
arrangement corresponding to the growth faces ({111}, {100}, {110}). - CVD synthetic diamond: often orange-red or green striped fluorescence layers
parallel to the growth direction; characteristically layered.
Willems et al. (2011) describe luminescent regions in CVD diamond directly observable
under DiamondView conditions: Gems & Gemology 47(3), 202–207
(DOI: 10.5741/gems.47.3.202) [VERIFIED].
Wang (2007) reported on latest-generation CVD-grown synthetic diamonds and their
fluorescence character: Gems & Gemology 43(4), 294
(DOI: 10.5741/gems.43.4.294) [VERIFIED].
Note: detailed DiamondView instrument specifications are proprietary and not available
in open peer-reviewed literature. The descriptions above are synthesised from the
peer-reviewed papers cited (Willems 2011, Wang 2007) and Gem-A Diploma teaching
materials. Willems et al. and Wang et al. are inferred use references – their papers
describe CVD diamond fluorescence as observed under DiamondView-like conditions.
DiamondView Limitations
- Does not work reliably on very small melee diamonds (below ~0.05 ct) where growth
pattern detail is unresolvable. - Cannot be used on stones set in jewellery – requires all-round UV illumination of the
bare stone. - Training is required to interpret fluorescence patterns reliably.
- Some natural Type IIa diamonds show unusual fluorescence that can be ambiguous.
DiamondSure (415 nm Spectral Screener)
Principle: DiamondSure (also a De Beers instrument) measures the optical absorption
spectrum of the stone, specifically checking for the presence or absence of the 415.5 nm
N3 absorption line (the Cape series diamond marker).
Most natural diamonds are Type Ia and show the N3 line at 415.5 nm. Stones that lack this
line – Type IIa diamonds, CVD synthetic diamonds – are referred by DiamondSure for further
testing. The instrument issues a binary result:
- "Pass" – stone shows the N3 line, consistent with natural Type Ia diamond.
- "Refer" – stone lacks the N3 line; further laboratory testing is required.
A "refer" result means the stone is unusual, not necessarily synthetic. Natural Type IIa
diamonds (including some of the finest D-colour colourless stones) will always give a "refer"
result. DiamondSure only screens – it does not confirm synthetic origin.
Note: the DiamondSure screening principle and its use of the 415 nm line are documented in
Gem-A Diploma materials and in peer-reviewed papers (Breeding & Shigley 2009, DOI:
10.5741/gems.45.2.96 [VERIFIED]); specific instrument documentation is proprietary.
Reflectance Meter (Reflectivity Meter)
Principle: A reflectance meter measures the percentage of normally incident light
reflected from a polished facet. At normal incidence, the Fresnel equation gives:
R = [(n − 1) / (n + 1)]²
for transparent non-absorbing stones, where n is the refractive index. Higher RI produces
higher reflectance.
This allows approximate RI estimation for stones whose RI exceeds the refractometer's
upper limit (~1.81) – making it useful for screening diamonds, moissanite, and high-RI
simulants at the trade counter.
Reflectance Reference Values
Values below are approximate; exact figures depend on illuminant and detector design.
| Species | Approximate R (%) | Notes |
|---|---|---|
| Diamond (RI 2.417) | ~17 | Highest R of any natural gem; typical thermal probe positive |
| Moissanite (SiC) | ~17 | Near-identical to diamond – cannot be separated by reflectance alone |
| CZ (RI ~2.15) | ~13–14 | Clearly below diamond; fails thermal probe |
| Zircon (high, RI ~1.93) | ~10–11 | Well below diamond; useful for stones above refractometer range |
| Demantoid (RI ~1.89) | ~10 | Similar to zircon in R |
| Glass (SG 2.0–4.2) | ~4–8 | Low R; confirms simulant for most lead-glass types |
Important Limitations
- Moissanite cannot be separated from diamond by reflectance alone – both give ~17% R.
The electrical conductivity test (see above) must be used first. - Polished surface quality critically affects the reading – a scratched, frosted, or dirty
facet gives erroneously low R. - Some instruments display an RI estimate computed from measured R using the Fresnel
equation. The accuracy of such an estimate is limited and was not confirmed in
retrieved primary literature during source verification – treat as approximate guidance
only; do not rely on it for definitive RI determination. - Hodgkinson (2016) reported anomalous reflectance meter behaviour on a Sumitomo
synthetic diamond – illustrating that synthetic diamonds can produce unexpected readings:
Journal of Gemmology 35(4), 274–275 (DOI: 10.15506/jog.2016.35.4.274) [VERIFIED].
When to Refer to a Laboratory
In-trade screening instruments (thermal/electrical probes, DiamondSure, reflectance meter)
are first-line tools only. Refer to a major gem laboratory when:
- A diamond probe gives "diamond" but the stone is suspected to be synthetic (especially for
high-value or unusually large stones). - DiamondSure gives a "refer" result.
- The stone shows unusual fluorescence under a standard SW-UV lamp (orange, chalky white,
or inert in a parcel that is otherwise predominantly blue fluorescent). - The stone shows strong birefringence under the polariscope (moissanite; diamond is
isotropic SR). - A blue or grey diamond is found in a parcel (Type IIb: electrical conductivity positive).
- Any stone of significant value where synthetic or treatment history is commercially relevant.
Sources
Key citations for this topic:
- Henn, U. (2021). "Synthetic Moissanite Testing as 'Diamond' Using Diamond Testers."
Journal of Gemmology 37(8), 778–779. DOI: 10.15506/jog.2021.37.8.778 [VERIFIED] - Speich, L., Chalain, J.-P. & Krzemnicki, M. S. (2022). "Synthetic Moissanite with the
Reflectivity of Diamond." Journal of Gemmology 38(4), 323–325.
DOI: 10.15506/jog.2022.38.4.323 [VERIFIED] - Hodgkinson, A. (2016). "Anomalous Behaviour of a Sumitomo Synthetic Diamond on the
Reflectance Meter." Journal of Gemmology 35(4), 274–275.
DOI: 10.15506/jog.2016.35.4.274 [VERIFIED] - 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 [VERIFIED] - Wang (2007). "Latest-Generation CVD-Grown Synthetic Diamonds From Apollo Diamond Inc."
Gems & Gemology 43(4), 294. DOI: 10.5741/gems.43.4.294 [VERIFIED] - Breeding, C. M. & Shigley, J. E. (2009). "The Type Classification System of Diamonds
and Its Importance in Gemology." Gems & Gemology 45(2), 96.
DOI: 10.5741/gems.45.2.96 [VERIFIED] - Read, P. G. (ed.). Gems 7th ed., chapter "Luminescent, electrical and thermal properties
of gemstones." DOI: 10.4324/9780080507224-17 [VERIFIED] - Martineau, P. M. et al. (2004). "Identification of Synthetic Diamond Grown Using Chemical
Vapour Deposition at Temperatures Below 1000°C." Gems & Gemology 40(1), 2.
DOI: 10.5741/gems.40.1.2 [VERIFIED]