Diamond Testing
How a lab tells natural from lab-grown.
Choose a stone, then run it through the seven screening tests gemmological laboratories actually use. Each test shows what the instrument sees — and how the reading differs between a natural diamond, a CVD-grown diamond and an HPHT-grown diamond.
Long-wave UV (365 nm) fluorescence Natural
Natural
When present: blue (N3 centre), even and concentric. About a third of natural diamonds fluoresce.
CVD
Weak or inert. Orange or yellow-green when present — often zoned or banded.
HPHT
Greenish-blue. A cross-shaped pattern may show on the crown or pavilion.
Phosphorescence after short-wave UV Natural
Natural
Effectively none — fades the instant the lamp goes off. Rare exception: Type IIb blues.
CVD
Uncommon. If present, weak and gone in under ~3 seconds.
HPHT
Strong greenish-blue afterglow, 3 to 60+ seconds. The hallmark tell.
Deep-UV growth imaging Natural
Natural
Concentric, often irregular growth zoning from octahedral growth.
CVD
Striated, layered bands parallel to the growth surface.
HPHT
Cross or hourglass cuboctahedral growth sectors.
Cross-polarised strain Natural
Natural
Mottled, irregular "tatami" smudges with rainbow interference colours.
CVD
Strong, coarse banded strain — layered cross-hatch shadows.
HPHT
Little to none — the view stays almost entirely dark.
FTIR absorption spectrum Natural
Natural
Aggregated-nitrogen peaks at 1014, 1082, 1282 and 1365 cm⁻¹; 3107 cm⁻¹ C–H.
CVD
No nitrogen peaks in the 1100–1400 range. May show 3123 cm⁻¹ if treated.
HPHT
No aggregated nitrogen. Boron peaks if Type IIb; otherwise nearly flat.
Photoluminescence (PL) spectrum Natural
Natural
415 nm N3 peak — the natural fingerprint.
CVD
No 415 nm. 737 nm Si-V doublet — the CVD fingerprint.
HPHT
No 415 nm. Variable NV centres; typically no Si-V.
Automated AI screening Natural
Natural
Pass — N3 peak, Type Ia nitrogen, no afterglow, concentric growth.
CVD
Si-V at 737 nm, striated growth, Type IIa.
HPHT
Persistent afterglow, sector growth, metallic flux.
Why these tests work
A lab-grown diamond is not an imitation — it is diamond. What differs is how the crystal grew: millions of years in the mantle, weeks in a CVD reactor, or days in an HPHT press. Each route leaves its own signature in the crystal — which trace atoms it picked up, how its nitrogen aggregated, how its growth layers stack, how much strain it carries. No single test is the whole answer; a laboratory reads several together, exactly as this page lets you do.
Want the full test-by-test reference behind this simulator? See how to tell if a diamond is lab-grown — the complete comparison table across all seventeen diagnostics. And for the two growth methods themselves, read lab-grown vs natural diamonds.
Now watch one respond to light.
Testing tells you where a diamond came from. Light tells you why it's beautiful. Explore any stone in real time in the free interactive demo.
Open the demo →