2007: How Diamond Graining Produces Hazy Graining On Diamonds?
One of the most confusing aspects of diamond grading has long been how graining affects the grade. In an effort to clarify its procedures, the Gemological Association of America has recently offered an intense technical report on graining in the winter issue of Gems & Gemology, and an abridged version released to editors.
Because the approach to graining, and its impact on grading, has changed throughout the years, the GIA aims to explain how, today, the lab treats the presence of graining.
Essentially, according to the GIA, while graining takes several forms and is less quantifiable than solid inclusions, the lab has set standards in deciding how graining affects clarity grading. To start, they outline three categories of graining:
Banded “whitish” graining that appears in an otherwise high clarity diamond
Hazy whitish graining resulting from submicroscopic particles
Reflective graining, a distortion in the diamond’s crystal structure that results in reflective lines appearing when the stone is tilted in a certain direction.
These forms of graining were ignored by the GIA in the 1960s, just generally noted in the 1970s, and rated by extent of graining in the 1980s. Thus, the confusion today over how graining impacts clarity.
These articles explain how graining is uniformly approached to ensure consistence in grading through the following guidelines:
Unlike most clarity characteristics, visibility at 10x magnification is not the sole determination. Rather, if a diamond shows graining, then factors like whether it can be seen from crown or pavilion, or ease of visibility, come into play.
Lighting plays a key role, and graders work under subdued fluorescent ceiling lights that bring out natural contrasts and minimizing surface glare
Graining is located with a darkfield gemological microscope, then examined with a 10x loupe and the microscope’s overhead light
To create a repeatable standard for grading when the graining is subtle, like a hologram that appears only when seen at a certain angle, graders hold the stone a set distance from light and angle it through a range of motion. Graining available straight through the crown or table is more likely to affect clarity grade than one visible only at an extreme angle.
Like any clarity characteristic, graining’s impact on grade rests in its size, nature, number, relief and location. These factors come into play in the following ways:
The graining’s affect on the diamond’s transparency, or how reflective the graining is, can lead to downgrading. Whitish graining that doesn’t alter transparency would have little effect, for example, but whitish haze visible through the crown would.
The greater the number of lines through the crown, the more likely that the clarity grade will be lowered.
How readily the grain lines or haze stand out from the diamond, and not just its visibility at 10x, but also the form and texture of the graining can affect the stone’s clarity grade. For example, a small sheen area that does not appear as bands may not affect grading, but if it takes the form of dark bands, the grade may decline, especially if visible through the crown.
It is our understanding from these reports that, despite the GIA’s claim that they have repeatable standards based on wide experience, this is a subjective process depending on the person doing the grading. In the final analysis, the impact graining has on the stone is determined not just by the GIA’s experts, but four other eyes: two of the stone’s seller and two of its buyer.