Handheld Testers and False Positives
A handheld tester provides rapid information precisely because it addresses a narrow problem. The error occurs when that narrow result is expanded into a claim the device did not measure.
If an instrument displays DIAMOND, the first question is not “is the stone natural?” but “what physical property did the device just measure, and how did it classify it?”
A Thermal Tester Measures Thermal Response
Conventional diamond testers make use of diamond’s very high thermal conductivity. The probe applies a controlled thermal pulse to a small contact area and monitors heat dissipation.
Such a test can separate diamond very effectively from many traditional simulants, including cubic zirconia. But thermal response is not a geological signature. Natural and synthetic diamond (laboratory-grown) have the same diamond lattice and can produce the same basic “diamond” response.
Moissanite Is the Classic Boundary of a Thermal-Only Approach
Synthetic moissanite changed the practical use of handheld testers because its thermal conductivity is high enough that conventional thermal-probe devices could react as though the material were diamond. This was well documented in GIA research as early as the late 1990s.
The lesson is not that every modern tester necessarily misidentifies moissanite. Devices differ. The lesson is that overlapping physical properties limit the selectivity of an individual test.
Electrical Conductivity as a Complementary Axis
Because of the moissanite problem, some handheld devices combine thermal and electrical response. This can improve separation, but electrical conductivity is not an absolute discriminator either.
The most important diamond exception is type IIb, in which boron can produce measurable electrical conductivity. The rule “conducts electricity = moissanite” is therefore not professionally valid.
A device must be interpreted according to its own algorithm, scope, and documented exceptions.
Diamond-Positive Is Not Natural-Positive
This is the chapter’s fundamental boundary.
If a tester confirms properties compatible with diamond, the result may correspond to:
- natural diamond;
- HPHT-grown diamond;
- CVD-grown diamond.
Synthetic diamond is not a simulant. A “diamond” result on a synthetic stone, from a tester designed to distinguish diamond from simulants, is therefore a correct result—not a false positive.
A false positive occurs only when the instrument or operator incorrectly places the sample in a category the test actually claims to distinguish.
Five Types of Problematic Result
In practice, it is useful to distinguish among:
- false positive — the test positively classifies a sample that does not belong to the target category;
- false negative — the sample belongs to the target category, but the test misses it;
- false refer — the sample is unnecessarily sent for additional testing;
- inconclusive/no read — the method does not provide a classification;
- out of range — the sample or conditions lie outside the device’s validated scope.
These categories are not the same as instrument failure. Sometimes refer or no read is precisely the correct safety response.
Probe Contact Is Part of the Measurement
The result can be affected by:
- a facet that is too small;
- contact between the probe tip and metal;
- a curved or inaccessible surface;
- contamination with oil, wax, or polishing residue;
- a thermally unstable sample;
- a damaged or dirty probe;
- a position outside the device’s stated operating range.
On mounted jewelry, a metal prong can be a particularly important source of artifacts. The operator must see where the probe is actually touching the stone.
Temperature, Surface, and Repeatability
A thermal test is a physical measurement, so environmental and contact conditions are not irrelevant. There is no single universal temperature or single universal procedure for every device; the specifications of the particular model apply.
When a result is unexpected, the rational sequence is to:
- inspect the surface;
- safely clean the sample, when appropriate;
- check the probe and device functionality;
- stabilize conditions if the sample is extremely cold or warm;
- repeat the measurement on a suitable facet;
- compare it with a reference sample in accordance with the instrument instructions;
- escalate to another method if the result remains disputed.
A repeatable result is not automatically an accurate result. An instrument may apply an inappropriate criterion to a sample outside its scope with great consistency.
Calibration, Functional Check, and Accuracy Are Not the Same
In everyday speech, “calibration” is often used for every form of device checking. Metrologically, it is better to distinguish among:
- formal calibration according to a defined procedure;
- an internal or manufacturer functional check;
- a test with a reference sample;
- an assessment of repeatability;
- independent validation of accuracy and classification scope.
A manufacturer’s marketing claim about an accuracy percentage does not have full evidentiary value without information about the test population, sizes, colors, simulants, mounted or loose condition, and definition of success.
A Handheld Property Tester Is Not a Spectroscopic Screener
Two families of devices must be distinguished.
A property tester measures one or several macroscopic properties, such as thermal and electrical conductivity. A spectroscopic screener uses an optical or luminescence signal to separate populations of natural and potentially synthetic diamonds.
Both groups can be handheld or compact, but their evidentiary logic is not the same. Programs and devices for natural/synthetic screening are covered in detail in Chapter 73.
Troubleshooting Matrix
| Situation | First question | Reasonable next step |
|---|---|---|
| “Diamond” on an unknown stone | did the device only confirm a property compatible with diamond? | confirm material identity; establish origin separately |
| “Diamond” on a sample suspected of being moissanite | is the tester thermal-only? | add an optical/microscopic or other independent method |
| “Moissanite” or an unusual electrical result on a blue diamond | could type IIb be conductive? | do not conclude from a single property test |
| unstable result in a setting | is the probe touching metal? | change the contact point or use a more suitable method |
| no read/refer | is the sample outside the scope? | verify conditions and escalate |
Chapter Summary
- A handheld tester measures a property, not “naturalness.”
- Thermal testers use diamond’s very high thermal conductivity.
- Synthetic moissanite historically demonstrated the boundary of thermal-only testing.
- Electrical conductivity can help, but type IIb diamonds are an important exception.
- Diamond-positive does not distinguish natural from synthetic diamond.
- A “diamond” result for a synthetic diamond is not a false positive if material identity is the purpose of the test.
- False positive, false negative, false refer, and inconclusive are not the same problems.
- Probe contact, setting metal, contamination, and temperature can create artifacts.
- Repeatability is not the same as accuracy.
- Model-specific sizes, ranges, and limitations must not be generalized to every device.
- A marketing claim about “accuracy” is meaningful only when the validation methodology is known.
- A handheld property tester and a spectroscopic screener belong to different evidentiary levels.
Sources for this chapter
Sources are primary or authoritative; the label in brackets gives the jurisdiction or source type.
- GIA / G&G: Nassau et al. 1997, Synthetic Moissanite: A New Diamond Substitute
- Natural Diamond Council: Strengthening consumer protection: ASSURE Program for the testing of Diamond Verification Instruments is expanded (27 September 2021) (industry program)
- Natural Diamond Council (PR Newswire): Natural Diamond Council launches the new edition of the ASSURE 2.0 Program online portal (21 November 2022) (industry program)