A technique I learned more than a decade ago is generally termed ‘water grading’ and is practiced in at least two of the major laboratories. Its primary purpose is to make diamond clarity grading easier and more precise. I also use it (or a variation discussed later in this article) when examining coloured gemstones. The tools needed are easy to acquire and very inexpensive. I couldn’t imagine grading a diamond without using this technique—and I don’t!
You’ll need a small container with a relatively wide mouth and tight-fitting lid, distilled water, dishwashing liquid, and a sponge-tipped applicator. The latter can be found in most stores that sell makeup and are used for applying eye-shadow, lipstick, etc. They come in a variety of sizes, although I prefer the smallest. You can also purchase much more durable and professional-looking applicators from medical supply companies. These feature a 6-in. long handle with a sponge tip about an inch long. The longer handle and more extensive sponge make them very versatile for manipulating a loose diamond in tweezers or microscope stone holder. The makeup applicators each last for a week or so; the more expensive ‘professional’ applicators can last for at least a month with proper care.
To begin, simply fill the container about halfway (it really doesn’t matter if it’s more or less than that) with distilled water and add a drop or two of dishwashing liquid to act as a ‘wetting agent.’ Since diamonds are hygroscopic, the water beads up on the surface without the dishwashing liquid. If you add too much, you may end up with annoying little bubbles or a soap film on the stone’s surface; not enough, and the water’s surface tension won’t be sufficiently broken to prevent beading. Once the solution has been made, it’s simply a matter of wetting the sponge, drawing it gently across the container’s rim to remove excess liquid, and then ‘painting’ the stone’s surface while examining it under the microscope.
If you’ve never used this technique, I’m sure you’ll be startled by the view inside the diamond. It’s as though the stone’s surface disappears, along with any dust; the water seems to act as an auxiliary lens.
It was several years after initially learning this grading technique that I was taught some of the subtleties of its use. As the sponge is drawn across the diamond’s crown, it is reflected in the pavilion. Against the light-coloured reflection, inclusions tend to appear dark, while dust or other objects on the pavilion surface disappear or appear light. Since we work primarily with mounted stones, cleaning can be problematic and this technique can significantly enhance our ability to differentiate inclusions from debris on the stone. Additionally, feathers reaching the crown’s surface will sometimes become reflective as the sponge is drawn to the opposite side of the stone.
When this technique is used to examine coloured gemstones, the water helps lessen the appearance of scratches and surface abrasion. When the gem is severely abraded, I may substitute a more viscous liquid with a higher refractive index (RI). Water’s RI is approximately 1.33, although the addition of the dishwashing liquid may alter that number slightly. Most common cooking oils, such as corn, canola, olive, etc., have refractive indices in the range of 1.44 to 1.48. While not a large numerical difference in RI, the higher indices significantly increase the effectiveness of the oils in reducing the visibility of surface abrasion. I usually start with the water solution because it is easily cleaned from the stone. However, if it doesn’t provide the necessary visibility, I switch to a higher RI liquid. If cooking oil is insufficient, benzyl benzoate is non-toxic, has a mild odour, and an RI of approximately 1.567. It is also sold under the trade name, ‘Refractol.’ This is, to my knowledge, the highest RI available in a non-toxic liquid and I’ve used it to great advantage for wet grading and when immersion microscopy was necessary. Of course, wet grading is always most effective with proper illumination.