Professional Jeweler Archive: Checking for Fakes

June 2000

Diamonds/Gemology


Checking for Fakes

Luster, fracture and double refraction are a few of the visible clues when checking whether a stone is really a diamond


A lot of stones are diamond wannabes. Or more correctly stated, their owners want to pass them off as diamonds even if they’re white sapphire, cubic zirconia, synthetic moissanite, zircon or other material.

When new jewelry arrives in the store or when a customer brings in a piece for repair or valuation, how can you be sure the stone is a diamond? The first line of defense is a diamond tester that checks thermal conductivity. Because synthetic moissanite has thermal characteristics similar to diamond, you may want to buy a special tester for it.

Getting a headache yet? Don’t worry. Here are a few visual clues to help you single out simulants. If these tests raise suspicions, other gemological tests may be required. If you’re still not sure, send the stone to a reputable gem lab.

Luster

A diamond’s luster (the return of reflected light from the polished surface of the stone) is often described as adamantine, which is just a notch below metallic luster. A stone’s luster is determined by its refractive index and how well it’s polished.

Because of diamond’s durability and hardness, its luster can remain adamantine through decades, even centuries of use. If a diamond has been mistreated, however, it can show some fracturing along facet junctions. Even so, a diamond’s facets in general remain sharp.

In gemstones other than diamond, all of which are softer, the facets can abrade and the flat surfaces can be scratched from even normal wear. These marks interfere with a stone’s luster.

The lesson for retail jewelers: be careful to examine all preowned stones along the facet edges to check for signs of wear.

Cleaving and Bearding

Diamonds tend to cleave (separate along planes of atomic weakness) rather than fracture. When diamonds do fracture, the break is often jagged and step-like. In all other stones, fracture patterns are different. Caution: synthetic moissanite can show cleavage patterns and step-like breaks so look for other clues (see “Single or Double Refraction” section).

Bearding refers to microscopic, hair-like feathers along the girdle edge that radiate into a diamond. They sometimes occur as a result of bruting (a process in which rough diamonds are given an outline shape). No diamond simulant shows bearding.

As an added visual test, note whether the stone has a faceted girdle. Not all diamonds have faceted girdles, but those that don’t have sugary, granular girdles that aren’t seen in the simulants.

Single or Double Refraction

Refraction describes the way light travels in a gem and can be best seen through a microscope. With singly refractive gems such as diamonds, you can look through a large facet and see no doubling of facets on the opposite side. Each facet edge appears single and sharp.

Gems with double refraction – such as corundum, zircon and synthetic moissanite – split light into two rays that bend and travel through the gem at different speeds. If you look through a large facet, you see a doubling of facet edges on the opposite side. In gems with pronounced double refraction, such as zircon and peridot, the gems can even look hazy.

One caution: cubic zirconia, like diamond, is singly refractive, so you may have to use a diamond tester in conjunction with checking the diamond’s refractive index.

– by Robert Weldon, G.G.

Diamonds retain superb luster even after centuries of wear. This diamond was mounted in Spanish Colonial jewelry circa 1780 and still has sharp facet edges and adamantine luster.
This natural, colorless zircon shows a less-than-adamantine luster. Note the abraded facet edges.
Most diamond simulants have a glassy or vitreous fracture that may appear shell-like, much like broken glass. Example at the top right is a cubic zirconia.
Minute feathers – or bearding – extend from the girdle edge into the diamond at left. No diamond simulant has feathers of this kind.
When diamonds fracture (which is rare), the break is jagged, often step-like as in the example.
Diamonds are always singly refractive, showing only one set of facet edges. But you shouldn't consider single refraction to be diagnostic by itself because some diamond simulants are singly refractive also.
Some diamond simulants are doubly refractive, showing double sets of facet edges.
All photos by Robert Weldon

Copyright © 2001 by Bond Communications