Diamond Type, and What “Type IIa” Actually Tells You

A diamond’s type is a classification of what sits in its crystal lattice besides carbon, chiefly nitrogen and boron, read on an infrared spectrometer. It is not a quality grade and not one of the 4Cs. It is chemistry, measured with an instrument.

Blue nuance in a colorless laboratory-grown diamond is usually boron: in its 2017 review, GIA found that a majority of colorless HPHT samples were Type IIb, and boron at those levels can lend a faint blue cast without shifting the color grade — which is why type classification tells you something a color grade does not.

This Operia guide sets out the four types, why laboratory growth inverts what nature produces, where blue nuance comes from, and what “Type IIa” on a listing does and does not mean.

What are the four diamond types?

Diamonds are sorted by whether they contain nitrogen, how that nitrogen is arranged, and whether boron is present instead.

Arrangement matters as much as quantity. A lone nitrogen atom absorbs light across a wider band than the same atoms locked into a pair, which is why Type Ib stones show a more intense yellow, while the paired nitrogen of Type IaA doesn’t affect color at all.

Note the word measurable. Type IIa doesn’t mean a diamond contains no nitrogen at all; it means the infrared spectrometer can’t detect any. The classification is set by the instrument’s threshold, not by absolute purity.

What does a reactor make that geology rarely does?

Type IIa. Nature makes it rarely: 1–2 percent of natural diamonds. Laboratory growth turns that around. CVD produces Type IIa as its ordinary result, and both of the largest polished laboratory-grown diamonds, the 75.33-carat Celebration of India and the 50.25-carat Shiphra, were graded by IGI as Type IIa.

What geology almost never makes, a CVD reactor makes by default. That single inversion explains most of what follows, including why “Type IIa” on a lab-grown listing means something different from the same words on a natural stone.

Which types do CVD and HPHT produce?

CVD: typically Type IIa

CVD diamond grows from a mixture of hydrogen and methane, so there is little nitrogen to begin with. Some growers add a small amount to the gas. GIA’s review of CVD-grown diamonds found that nitrogen in such stones generally sits below the detection limit of infrared spectroscopy, so they still classify as Type IIa. The record stones in our guide to how large laboratory-grown diamonds are made are all CVD.

HPHT: three groups

GIA’s 2017 review of HPHT-grown diamonds sorted them by their infrared spectra into three patterns:

What causes blue nuance?

Blue nuance is the faint turquoise, blue or gray cast that some colorless laboratory-grown diamonds show, and it is one of the complaints buyers raise most often. Its usual root is the boron in those Type IIb HPHT stones.

In a colorless HPHT stone, the boron sits at levels too low to shift the color grade, but not always too low to see. The D-to-Z scale measures how much yellow or brown a stone shows, and it has no place to record a faint blue. A stone can hold a colorless or near-colorless grade and still carry the cast.

Buyers who know what to look for describe it precisely. One, reading a listing, wrote: “That one is definitely a Type IIb that will appear light blue/turquoise due to boron.” Another, of a seller’s imagery: “Blue nuance is very visible in their pics and videos.”

The same boron fools testers

Type IIa has the highest thermal conductivity of any diamond type, so a thermal tester reads a CVD stone as diamond without hesitation. The half-remembered stories of testers misreading lab-grown stones trace to boron instead. Boron makes a Type IIb diamond conduct electricity, multi-testers use electrical conductivity to flag moissanite, and so a boron-bearing HPHT stone can read as moissanite.

What to check

Is “Type IIa” a mark of rarity on a lab-grown stone?

No. It is accurate, and it is worth knowing: it tells you the stone has no measurable nitrogen or boron, and so none of the boron behind blue nuance.

But in natural diamonds, Type IIa marks real scarcity, 1–2 percent of stones. In CVD it is the ordinary outcome of the process. A listing that uses “Type IIa” to suggest a lab-grown stone is rare borrows a meaning from natural diamonds that doesn’t carry over. We would rather say that plainly than have you learn it somewhere else.

Nor is it a quality grade. Type says nothing about how well a stone is cut, and a Type IIa stone can carry any clarity grade.

What Operia publishes

[OPERIA TO CONFIRM: whether Operia’s reports carry type classification]

Type tells you something the 4Cs don’t, so it belongs beside them. For every stone, we show:

Shop Diamonds|Read The Operia Standard

Frequently asked questions