Introduction to Diamond SimulantsIntroduction to Diamond Simulants

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Introduction to Diamond Simulants

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Distinguishing diamonds from their lookalikes is a prerequisite to diamond grading. Assigning color, clarity, and cut grades to a gem does no good if you can't tell if it's a diamond or a simulant.

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engagement ring with diamond and cubic zirconia - distinguishing diamonds
Engagement ring with a round 0.53-ct diamond center stone and round and princess-cut cubic zirconia accent gems. Photo by CustomMade. Used with permission.

In theory, you can distinguish a diamond simply by looking at it. No other gem has properties that exactly match a diamond's. However, this becomes difficult when examining set stones. Often, you can't see enough to make an accurate assessment. In addition, today's simulants imitate diamond's appearance much more closely.

Distinguishing Diamonds by Their Appearance

You can't make a positive diamond identification with your eyes alone. Nevertheless, the following features will often tell you if a gem is a diamond or not. Gemologists must educate their eyes by looking at many gems. You won't always be in your lab when presented with a prospective diamond.

Look for these features as you examine diamonds and other gemstones.


The ability to separate white light into rainbow hues, dispersion is one of diamond's most endearing qualities. How much dispersion, or fire, you see from a gem depends on its optical properties and cut. Although not all diamonds show the same amount of fire, the majority behave within a limited range.

Other gems usually show much more or much less dispersion. As you become familiar with diamonds, you'll learn to distinguish quickly gems that have too much dispersion, like cubic zirconia (CZ). Too little dispersion can only be determined after examining the cut. Don't expect much fire from a poorly cut diamond. However, if you have a reasonably well-proportioned gem with little fire, you know you have something other than a diamond.

Sharp Facet Edges

Due to a combination of extreme hardness and cut, diamonds have sharper facet junctions than other gems. The trick to determining a facet edge's sharpness is just a matter of observation. For example, CZ gems have high hardness (8-9). The difference is often too close to make a positive identification. However, most CZs aren't very well cut. While sloppy cutting doesn't prove a gem isn't a diamond, it's a good clue you're looking at an inexpensive gem.

Most diamond imitations are inexpensive and receive quick cuts. Be aware, however, that custom faceters take pride in making facet edges as sharp as possible, even for diamond simulants. You'll occasionally see some of these gems. The best are nearly impossible to distinguish from diamonds.

Diamond Edges

Here, you see a typical diamond showing off its sharp facet edges. (Even so, this is a low-quality gem with an L color, SI2 grade). Most diamonds typically receive such a cut.

diamond - sharp facet edges

To judge the cutting, begin with the star facets. Note that they all have the same size and proportion. Next, look at the facets to which they connect. You can see they're also in proportion to the other facets in the row. All the meets form very sharp points.

Cubic Zirconia Edges

Below, you see a CZ. If you look carefully, you can see the facet edges aren't quite as crisp as on the diamond. Nor is it quite round. It bulges towards the bottom of this picture.

CZ - facet edges

More significantly, look at the sloppy cutting. The star facets are large semicircles, not nice, neat little triangles. Some of the pavilion main facets reach the girdle, others don't. Compare that to the uniform facets on the diamond above. You'll see quite a difference.

Furthermore, one facet edge near the upper left has a serious abrasion. Some of the top right facets have pits. This is quick cutting. For a dollar, it looks good but it's not in the same category as a diamond.

Sapphire Edges

Take a look at this white sapphire. Here, the difference in edge sharpness is more obvious. Look at the star facets and the facets to which they connect. The facet sizes aren't uniform. The points don't all meet in the same places nor are they all sharp.

white sapphire - facet edges

This is another example of commercial cutting. While much better than the CZ, it doesn't reach custom or diamond cutting levels.


The amount of light reflected from the surface of a gem, luster results from refractive index (RI) and polish. Diamonds and other gems with RIs over 2.4 will have adamantine (mirror-like, "diamond-like") lusters. Essentially, they reflect more light than lower RI gems. Again, you must educate your eyes by becoming familiar with diamond luster.

The pictures illustrating facet edges above can give you some idea of luster as well. For example, the diamond has an adamantine luster. While the CZ has a sub-adamantine luster, the sapphire shows a vitreous, "glass-like" luster.

Read Through

An RI test can usually let you make a quick gem separation. However, diamonds and many of their lookalikes have RIs above the limits of most refractometers. For distinguishing diamonds, you can use an alternate method for comparing RIs.

Lay the test gems face down on a piece of printed paper. With most stones, you can see the print through the stone. This is called the "read through" effect. With high RI simulants, you can only see a little bit of type. However, diamonds don't allow any of the text to show through.

distinguishing diamonds - read through test
A read through test can help distinguish diamonds from imitations with lower refractive indices.

Keep in mind that no gems with RIs equal to or higher than diamond will allow read through. For example, moissanite, with an RI of 2.6, will test the same as diamond. The same holds true for sphalerite, cadmium sulfite, rutile, and a few other rare gems. Thus, this test works best for distinguishing diamonds from lower RI simulants.

Also note, the read through effect only works with well-cut gems. You can read through poorly cut stones, regardless of their RI.


Often, inclusions offer a quick way to separate diamonds from lookalikes. The most common inclusions unique to diamonds are: bearding, planar graining, naturals, trigons, and laser drill holes. Crystal inclusions and feathers and veils are common to diamond and many other gems. However, in combination with other factors, they can help you identify a diamond. For more information, consult the inclusions section of our "Clarity Grading Diamonds" article.


A diamond's girdle is usually waxy or granular. You won't find these features in many other gems, especially among those that closely resemble diamonds. Of course, you must educate your eyes to make this distinction. Sometimes, you'll find a diamond with a faceted and polished girdle. So, that doesn't eliminate a gem from being a diamond.

The picture below shows a common, granular diamond girdle.

distinguishing diamonds - granular diamond girdle

The next picture shows a CZ girdle. Note how it's glassy, not waxy or granular. CZs may have a lot of pitting. However, you'll find the pits always in a semi-polished area.

distinguishing diamonds - glassy CZ girdle

Bearding inclusions indicate a diamond. The picture below shows a diamond girdle with bearding. Note the cube near the top. That is a typical diamond inclusion.

distinguishing diamonds - diamond girdle with bearding

Diamond Simulants

Dozens of gems can simulate diamonds. Some do a much better job than others. Based solely on properties, gadolinium gallium garnets (GGG) come closest. However, some of the best simulants I've seen personally were some small, white spinel stones. Even though they had no dispersion, they were so brilliant it was easy to believe they were small diamonds. It took a careful look to distinguish these gems.

New simulants constantly enter the market. Therefore, you must conduct your examinations diligently. Just a bit of carelessness and you could make a mistake in the thousands of dollars!

Below, you'll find descriptions of the most commonly encountered diamond lookalikes. Of course, this doesn't include every possible imitation. Any colorless material can be (and probably has been) used at some time.

Cubic Zirconia

Cubic zirconia (CZ) gems rank as the most popular diamond substitute, primarily because they're inexpensive to manufacture. Their most distinguishing feature is their extreme dispersion: 0.058-0.066, compared to diamond's 0.044. A well-proportioned CZ will show much more fire than a diamond. Often, a sloppy-cut CZ will seem equivalent to a well-cut diamond, but the poor proportioning will give it away.

Usually, the cutting serves as an excellent clue (but not proof) you're looking at a CZ or other simulant. Diamonds usually have good to excellent pointing and polishing. (Very tiny or included diamonds are exceptions). Inexpensive simulants receive quick cuts, which you can easily see. Keep in mind that CZs sometimes get custom cuts. Distinguishing diamonds from such gems by cutting alone can be difficult. However, their dispersion serves as a giveaway.

When loose, you can easily separate CZs from other colorless gems by their high specific gravity (SG). Although usually clean, CZs may contain inclusions such as gas bubbles, partially or completely filled with unmelted zirconium dioxide powder. You can distinguish this powder due to its opacity and irregular shape.

distinguishing diamonds - inclusions in cubic zirconia

German Formula Lab Gems and Russian Jewels

Two relatively new CZ variations have entered the market: German Formula Lab Gems and Russian Jewels. These are essentially CZs with added carbon. In addition, Russian Jewels receive a diamond-like carbon (DLC) coating. Supposedly, these gems have reduced dispersion values closer to that of diamonds. I can't assess the accuracy of that statement because the samples I've tested were too varied in shape and cut. However, they all have very high dispersions, more than a well-cut diamond. When immersed in methylene iodide, as shown below, the stones showed a slight but apparent difference in RI. (Please note: methylene iodide is toxic. Follow the directions when using this liquid and use caution).

distinguishing diamonds - diamond, CZ, German Formula Lab Gem, and Russian Jewel immersed in methylene iodide

I couldn't determine any difference in hardness, even with the DLC coating, between these gems and other CZs. If present, it was too thin to make any difference. I found their inclusions, SG, and reactions to ultraviolet light the same as standard CZs.

These new gems possess no properties that distinguish them from other CZs. So, for practical purposes, identify them simply as cubic zirconia.

Gadolinium Gallium Garnet

Popular in the 1960s and 70s, gadolinium gallium garnet (GGG) was mostly displaced as a diamond simulant by CZ. This was largely due to CZ being less expensive to manufacture. With a single RI and a dispersion close to diamond, GGG gems are very difficult to distinguish by sight if cut well.

However, GGG has a hardness of 7, well below diamond. Therefore, these gems won't have facet edges as sharp as diamonds. Their luster ranks as vitreous, or at best sub-adamantine, rather than adamantine. If you can test a GGG for read through or SG, you'll notice significant differences from diamond.


Although common, glass makes a poor diamond substitute. A quick observation will find it shows much lower brilliance. Under magnification, glass can show inclusions such as air bubbles and swirl lines.

With the addition of lead, glass's dispersion can reach the level of diamonds or even higher.


Synthetic moissanite is a relatively new diamond simulant. (Natural moissanite is too small and rare to facet). Chemically, it's silicon carbide, a common abrasive. (Strictly speaking, only the transparent variety is new).

Moissanite has one optical property that differs significantly from diamond: double refraction. It has a birefringence of 0.043. In most cases, you can see strong doubling of the facet edges. However, this is only apparent if the gem is cut to the A or B axis. If cut to the C axis, you won't see any doubling through the table. You can see it from the side but be careful. Don't confuse reflections through the facets for doubling.

The picture below shows strong doubling of the pavilion main facets. Look for fuzzy edges and doubled facet lines, like those to the lower left. Also, note the high dispersion. You wouldn't find dispersion this extreme in a diamond.

distinguishing diamonds - moissanite strong doubling

Due to its high birefringence, moissanite's dispersion varies from 0.009 to 0.104. Faceters usually cut these gems with an orientation that gives them more dispersion than diamonds when viewed face up. When examining a moissanite, you'll always find a direction with extreme dispersion.

Colorless Gems

At one time or another, almost all colorless gems have been used to imitate diamonds. The usual suspects include colorless synthetic spinel, sapphire (both natural and synthetic), and topaz. This is due to the fact that these are the most abundant colorless gems. You may also come across tourmaline, goshenite (colorless beryl), quartz, or any other colorless stone.

In melee sizes, these stones can make a fair diamond substitute. However, in larger stones, they noticeably fail in brilliance and fire. A careful examination will distinguish them easily. They have rounded facet edges. Furthermore, their brilliance, dispersion, and luster all fall well below that of diamond.

Yttrium Aluminum Garnet

Yttrium aluminum garnet (YAG) was the first synthesized garnet available on the jewelry market. As far as diamond simulants go, GGG marked a distinct improvement. However, like its synthetic sibling, YAG's popularity decreased with the advent of inexpensive CZ.

Distinguishing diamonds from YAGs proves rather easy. Look for YAG's low refractive index, vitreous luster, rounded facet edges, and low dispersion.

Distinguishing Diamonds from Simulants by Their Properties

You can separate some of the most frequently encountered simulants from diamonds by comparing their SG range and birefringence. This comparison chart can assist you.

Familiarize yourself with the properties of these common lookalikes. This will help you make quick distinctions, without reverting to your instruments. Study this detailed chart of properties.

Diamond Testers

In theory, you should be able to distinguish a diamond from a lookalike visually. In practice, you'll find this rarely possible. Often, settings hide enough of the stone that you can't make a positive identification. Even when you can see the entire stone, making an identification solely based on a visual inspection is risky.

Instruments called diamond testers can not only help with distinguishing diamonds but also reduce the tedium of visual inspections. Released shortly after the introduction of CZs, the first devices measured thermal conductivity, which made it possible to distinguish diamonds from CZs. However, when moissanites emerged as popular diamond simulants in the 1990s, these devices couldn't distinguish them from diamonds because their thermal properties were so similar. As a result, a new generation of diamond testers began to include electrical conductivity tests. This can help distinguish most diamonds from moissanites.

Distinguishing Diamonds: Conclusion

Distinguishing diamonds starts with educating your eyes. With experience, you can frequently tell if a gem is a diamond or not with a simple loupe examination. That may not seem important right now, but the implications are far reaching.

Whenever possible, use a diamond tester to verify your observations. However, you never know when or where you'll be offered a diamond. If this occurs at an auction, yard sale, flea market, or pawn shop, you won't always have your tools handy. Great deals appear when you least expect them.

Notes on Diamond and Carbon Use in Industry

Due to the different ways carbon atoms can bond to each other and to other elements, carbon is versatile. Both graphite and diamond are carbon. In graphite, carbon atoms bond strongly to each other within a plane but weakly between adjacent planes. Graphite is soft, electrically conductive, and opaque. In diamond, the bonding is much stronger, even in the direction of cleavage. Diamond is the hardest known material. It has a coefficient of friction less than that of Teflon and extremely low chemical reactivity. It has thermal conductivity four times that of copper. In addition, diamond is electrically insulating and transparent from the ultraviolet to the infrared light range.

Not surprisingly, due to these properties, carbon has many industrial uses. Relatively recently, three methods have been developed for applying diamond-like coatings to tools and other items. Relatively inexpensive to apply, these coatings are becoming very common. You'll find them on razor blades, eye glasses, windows, electronics, data storage devices, and many other places where wear protection or low friction is critical.

Diamond-Like Carbon

An amorphous carbon coating can be produced in which a proportion of the carbon atoms bond as in diamond. These coatings have most of the physical properties of diamonds, although to a lesser degree. Since they resemble diamond in many ways, they are called diamond-like carbon (DLC) coatings.

Since DLC coatings can be applied at low temperatures, they're widely used on a variety of materials, including plastic and glass.

They possess many useful properties, such as:

  • Low friction
  • Scratch resistance
  • Very hard yet flexible
  • Chemical inertness
  • Corrosion resistance
  • Atomically dense
  • Diffusion resistance
  • Biocompatibility (suitable for internal use)

Chemical Vapor Deposition

Chemical vapor deposition (CVD) films are a polycrystalline form of diamond. Thus, they have stronger molecular bonding and are closer to diamond in properties than other films. However, CVD films require high temperatures (900° C to 1000° C) for application. Since many materials can't withstand these temperatures, this is the primary disadvantage to CVD films.

CVD films can be made up to 1.5 mm thick on substrates up to 12 inches in diameter. The diamond can remain on the substrate or be removed. When separated, it can be polished and laser cut into finished parts. In 2008, a new variation of this process allowing for nearly unlimited size was announced.

Gem-quality CVD diamonds have also been synthesized.

Composite Diamond Coating

Used primarily on metals, composite diamond coating (CDC) can be built up between 0.0003 and 0.010 inches in height. Such coated metals have much higher wear resistance, extremely low friction, and chemical and corrosion resistance. In addition, they can be made sufficiently conductive for applications requiring anti-static materials.

Donald Clark, CSM IMG

The late Donald Clark, CSM founded the International Gem Society in 1998. Donald started in the gem and jewelry industry in 1976. He received his formal gemology training from the Gemological Institute of America (GIA) and the American Society of Gemcutters (ASG). The letters “CSM” after his name stood for Certified Supreme Master Gemcutter, a designation of Wykoff’s ASG which has often been referred to as the doctorate of gem cutting. The American Society of Gemcutters only had 54 people reach this level. Along with dozens of articles for leading trade magazines, Donald authored the book “Modern Faceting, the Easy Way.”

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