Lab Notes Gems & Gemology, Summer 2017, Vol. 53, No. 2

Diamond with Concentric Inclusions

Diamond with concentric octahedral growth layers.
This 2.34 ct Fancy Intense yellow-green diamond (left) contains abundant inclusions that outline concentric octahedral growth layers. Through the table, the layers resemble nested rhombuses (center, field of view 3.54 mm). The inclusion shapes appear to be controlled by the layering, as seen through the pavilion (right, field of view 1.99 mm). Photos by Evan Smith.

As a diamond grows, layer by layer, it may incorporate small pieces of the surrounding mantle rocks as mineral inclusions. The three-dimensional arrangement of inclusions in most diamonds has the appearance of being somewhat random, with no discernible pattern. However, GIA’s New York laboratory recently examined a noteworthy exception.

A 2.34 ct Fancy Intense yellow-green type IaAB diamond was found to have abundant silicate inclusions with a more systematic pattern delineating growth layers (see above). Collectively, the inclusions outline con centric octahedral layers. The growth layers are made even more noticeable by a sort of optical graining that could be caused by layered distortions in the diamond crystal from variations in nitrogenconcentration. Many of the individual inclusions have an elongate or flattened oblate shape that conforms to the growth layering (bottom image). Thus, both the shape of individual inclusions and the collective distribution of multiple inclusions imitate the geometry of concentric octahedral diamond growth. This kind of symmetrical, growth-outlining texture is rarely observed in diamond.

The vibrant orange and green inclusions were identified as almandine-pyrope garnet and omphacitic clinopyroxene, respectively. Such mineral inclusions are distinctive of diamonds grown in an eclogite host rock, one of two common diamond rock types in the lithospheric mantle, the other being peridotite (S.B. Shirey and J.E. Shigley, “Recent advances in understanding the geology of diamonds,” Winter 2013 G&G, pp. 188–222). Diamonds like this one with eclogitic inclusions are sometimes called E-type, while those with peridotitic inclusions are denoted as P-type.

This is an interesting sample from a geological perspective. There are still many unanswered questions about exactly how diamonds form. Detailed analysis of this sample could provide valuable clues as each layer captures successive chemical snapshots of the diamond growth process. Of particular interest here is the question of how long it takes to make a diamond. It is possible that this diamond grew over the span of millions of years and the inclusions serve as an encrypted record of the process.

Evan M. Smith is a postdoctoral research associate, and Wuyi Wang is director of research and development, at GIA in New York.