Daniel Howell Ph.D.
Chief Scientist and Co-Founder,
Diamond Durability Laboratory

Nitrogen is almost ubiquitous in diamonds. So much so that its rare absence is highly sort after in gem diamonds. Some estimates1 suggest that over 98% of diamonds contain detectable amounts of nitrogen. As a result, diamonds are even classified based upon the characteristics of the nitrogen that they contain. The valuable colorless Type II diamonds have nitrogen contents of about less than 10 parts per million (for every 1 million atoms of carbon that make up the diamond, less than 10 are nitrogen), while Type I diamonds have been reported to contain up to 5000 ppm (0.5%; average values ~200-300 ppm).

While nitrogen is responsible for yellow color in diamonds, it is how the nitrogen is bound in the diamond that causes the color. When nitrogen is incorporated into a diamond, a single nitrogen atom takes the place of a single carbon atom. Diamonds containing nitrogen in this form are classified as Type Ib. They exhibit yellow color because the single nitrogen atoms absorb light in the violet and blue portion of the spectrum, resulting in our perceiving the color yellow.

In the old world this type of color was commonly termed canary yellow because of its unique high yellow saturation. More recently, the term Zimmi has entered into the gem industry to refer to these yellow Type Ib diamonds. The reason is that the Zimmi mining area of southern Sierra Leone has become an important source for this type of diamonds. However, Type Ib yellow diamonds can also come from other places.

At the high temperatures and pressures that diamonds experience deep in the Earth (~1000 – >1200oC), the single atoms of nitrogen are mobile within the diamond lattice. This means they can move around until they meet another nitrogen atom, changing the diamond’s classification from Type Ib to Type IaA. This pair of nitrogen atoms does not absorb light like the single atoms, and therefore the yellow color is lost. This process of nitrogen moving around the crystal and pairing up is called nitrogen aggregation, and is dependent upon three factors; (i) the amount of nitrogen in the diamond, (ii) the amount of time the diamond resided within the Earth, and (iii) at what temperature it resided. Given the high temperatures noted above, the time taken for the nitrogen to aggregate into pairs is very short, commonly less than 1 million years. When we consider that scientific research into diamond ages show that they can reside in the Earth for as little as 1 million years, but also for billions of years, it is clear to see why Type Ib diamonds are rare. A study by GIA2 from 2005 further underscores this rarity. They reviewed more than 10,000 yellow diamonds submitted to GIA in two non-sequential years; Type Ib diamonds represented only 0.8% of them.

Two more recent studies by GIA scientists3,4 has shed light on Type Ib diamonds specifically from Zimmi, adding a whole new dimension to their rarity. They reported total nitrogen concentrations ranging from 8 ppm to ~340 ppm, with the amount of single nitrogen atoms ranging from 8 to ~40 ppm. The nitrogen aggregation model suggests that these diamonds either had a very short residence within the Earth, or once they grew, they were moved to a shallower (and therefore cooler) part of the Earth and resided there for potentially a long time. Given that from a geological perspective the likelihood of this second option is thought to be rare, many diamond scientists would lean towards the first conclusion. In the case of these Zimmi diamonds, the first conclusion would appear to be wrong.

Dr Karen Smit of the GIA has just published data [4] from sulphide inclusions within Zimmi Type Ib diamonds that shows that they are ~650 million years old. Considering that the volcanic eruptions that brought these diamonds to the surface occurred around 150 million years ago, their lack of aggregation suggests that very soon after the diamonds formed, they would have beentransported to a shallower and cooler (less than 850oC) depth in the Earth. They would have resided there for nearly 500 million years before being brought up to the surface in a volcanic eruption.

The fact that the nitrogen aggregation process changes the single nitrogen atoms responsible for yellow color in Type Ib diamonds in a very short time at high temperatures deep in the Earth, combined with the diamonds’ vast age, means that this type of yellow diamond is very rare in nature. The initial hypothesis that these diamonds did not reside in the Earth for long, and can be considered a young relative to the more ancient diamonds is not a bad one. Yet the recent GIA study on Type Ib yellow diamonds from Zimmi shows that this might not always be the case, and that they are potentially even more special. They have survived residing in the shallower Earth for half a billion years without turning to graphite while also retaining their valuable color. This is quite a feat, and highlights another reason why this type of yellow diamond is so unique.

1 Shigley, J.E., 2005. Elements, 1, 101-104.

2 King et al., 2005. Gems & Gemology, 41, No. 2, pp. 88–115

3 Shigley, J.E. & Breeding, C.M., 2013. Gems & Gemology, 49 (4), 259-260.

4 Smit, K., Shirey, S.B. & Wang, W., 2016. Type Ib diamond formation and preservation in the West African lithospheric mantle: Re-Os age constraints from sulphide inclusions in Zimmi diamonds. Precambrian Research, in press.