Surface and internal graining are characteristics of diamond that very few people actually have a good idea of what it’s all about. The first thing you need to know about graining is that it is natural and it occurs because of defects in the crystal structure of diamond.
In this article, I am going to be explaining what the differences are between surface and internal graining, how graining is graded, why it occurs, and how it affects the appearance of diamonds.
How Graining Affects the Appearance of Diamond
Officially there’s no guidance given to consumers about how graining affects the appearance of the diamond nor the extent to which it may be taken into account of in the clarity or polish grades.
All you really need to know is that graining is generally negative and in some cases can be so significant that it can alter the refractive index of the diamond. Graining is also indicative that there may be other defects in the crystal structure of the diamond that may affect the durability of the diamond such as internal strain.
The way graining affects the appearance of the diamond is by affecting its transparency. A diamond that is affected by internal graining is often described as being hazy. Surface graining can sometimes be visualised as colored bands or lines on the surface of the diamond.
In order to really understand graining, you need to have a basic understanding of crystal structure.
The molecules in any solid material, whether metal, mineral, ceramic, or organic (i.e. wood), are either organised in a crystal structure (crystalline) or are chaotic (amorphous).
When a crystalline solid forms, the process by which it forms is referred to as crystal growth. In diamonds, this growth occurs over thousands of years and is never perfect naturally. The reason is because the imperfections in crystal growth are caused by the Earth’s gravity.
Imperfections in a crystal structure are classified into point, linear, or planar defects. All you need to know about point defects is that they are essentially unoccupied places in the crystal structure where a carbon atom should be and this is where impurities can get into the diamond. Impurities such as nitrogen in diamonds are the causes of color and fluorescence in diamonds.
Linear defects are where there is a misalignment in the crystal structure. These misalignments cause atoms to be displaced. For those engineers and chemists out there, I’m referring to ‘dislocations’. For the rest of you, all you need to know is that these misalignments cause internal strain in the diamond.
Internal strain is invisible unless it is viewed under polarised light with a cross-polarisation filter. It is not recorded in a grading certificate and the only way a consumer can tell is by asking an experienced gemmologist to review the diamond. In diamonds of high clarity, internal straining is not likely to be so high as to affect the durability of diamond. However, in diamonds with SI cavities or feather inclusions that increases the impact of internal strain, then the diamond is much weaker at those points and may well be a durability issue.
An example of a planar defect is a grain boundary. Grain boundaries occur when the grains of two simultaneously forming crystals meet. When this happens, the direction that crystal is growing in essentially shifts.
There are two sub-categories of internal graining, reflective and whitish. Reflective internal graining occurs when dislocations occur at the edge of a grain boundary. These edge dislocations can slip along what is known as a slip plane. When the slipping of the dislocations is intense, it forms parallel bands known as slip bands.
Whitish internal graining occurs when the defects in the crystal structure changes the refractive index of the diamond such that it affects the crispness of the diamond. Sometimes cloud inclusions that are so small they cannot even be identified as individual pinpoints are considered as a type of whitish internal graining even though they are not related to crystal structure defects.
The term ‘surface graining’ is confusing because it isn’t immediately clear whether the graining occurs internally or externally. This is done on purpose because surface graining simply means that the effect of the graining occurs very near or at the surface of the diamond and can either be internal or external to the diamond.
Again, when reflective internal graining occurs so near to the surface of the diamond that it cannot be visualised as internal graining but can only be seen with reflected light or ‘glare’, then it is considered as surface graining.
Surface graining also occurs when there is a directional shift in the crystal growth that occurs at the surface of the diamond. The hardness of a diamond is directional depending on its crystal structure. When a diamond with this type of surface graining is polished, it leaves polishing/graining lines due to the difference in hardness at the point where the crystal growth has changed directions.
Finally surface graining can occur naturally in a diamond due to impurities in the diamond causing the crystal growth to occur at different rates. These impurities also make the diamond softer so that grain lines are visible after polishing.
A Brief History on Grading Graining
The consensus has always been that the presence of graining should negatively affect the price of diamond. However, in the early days of diamond grading, the experts were at a loss as to how to give graining a meaningful grade.
In the 1950s – 1960s, whitish graining was not noted on the grading reports at all.
In 1970, grading reports began to include comments about graining. For example, a 13+ carat, D-color diamond that had no other clarity defects apart from graining that was readily visible was graded IF in clarity but was remarked with a comment noting “Near transparent due to unusual internal texture.”
Another example from 1973: “Nothing generally regarded as a flaw or imperfection observed, however whitish internal graining is present.”
And in 1977: “Based on parallel whitish graining not shown [on the report diagram].”
By 1980, this whitish graining was graded just like other clarity characteristics at 10x magnification.
Nowadays, this takes the form of a comment: “The clarity grade is based on internal graining that is not shown”. Where graining does not affect the clarity grade, it is remarked with the comment: “internal graining is not shown”
A diamond with surface graining cannot be graded as having Flawless clarity, but an Internally Flawless grading can be given. If the surface graining is significant, a diamond can be given a VVS or below clarity grade even if there are no other clarity features. However, it is unusual for graining to affect the clarity grade below a VVS2.
The presence of internal and surface graining is one of the reasons why it is important when buying diamonds that you have a high quality image of the diamond, even better if you can get a high definition 360-degree video of the diamond.
The effects of graining may sometimes be confused with the effects of a highly fluorescent diamond when exposed to ultraviolet light. This is why if you want to evaluate graining in a diamond that exhibits fluorescence, you should view it under lights that do not emit ultraviolet light.
It is important to remember that diamonds with surface and internal graining that have been cut successfully will generally be safe for wearing. However, I would not recommend diamonds that come with comments stating that their clarity grade is because of surface or internal graining. These diamonds often have unusually clean inclusion plots for their clarity grade and may seem like great value for the uneducated. But as the old adage goes, “if something sounds too good to be true, it probably is”.