Investors Beware!
Surface-Treated Diamonds Increase Risk For
Those Who Can’t Spot Them

By Antoinette Matlins

By Antoinette Matlins, gemologist-consultant and author of many award-winning books, including Diamonds: The Antoinette Matlins Buying Guide: How to Select, Buy, Care for & Enjoy Diamonds with Confidence and Knowledge (GemStone Press)
Colorless and fancy color diamonds are the focus of investors today, but few are aware that many artificially colored diamonds are being sold without disclosure, sometimes accompanied by counterfeit GIA reports.

A variety of techniques are now used to transform off-color diamonds into diamonds that appear to have rarer and costlier colors: colorless and rare fancy colors.  Some use very sophisticated instrumentation; these are difficult and expensive to produce and the resulting colors are permanent.  There are also an increasing number of surface-coated diamondsmade by a much easier and cheaper method that involves applying a special coating to the surface of the diamond; the resulting colors include colorless grades (D-F) as well as “fancy” colors, but the color produced is not permanent, and should sell for a fraction of the cost of other diamonds. This is an area of serious concern because they are being sold as “natural color” diamonds. 

To complicate matters further, many diamond dealers and jewelers don’t have the knowledge and skill themselves to know whether or not a diamond has been coated to alter the color. Misleading terms have caused confusion as well. Treated-color diamonds reportedly treated by an infusion technique—the very term suggesting that the resulting color penetrates into the stone—have come into the marketplace. But these “infusion-treated” diamonds are off-color stones to which a surface coating had been applied—on the bottom of the stone only.
The number of such stones in the market is increasing. They are popping up in virtually every color—including shades of yellow, pink, and blue—and in every size, shape, and clarity.

But Have No Fear….A Simple Carbide Scriber* Can Solve the Mystery!

Fortunately, this particular treatment is one that is easy to spot, with a tool called a carbide scriber. By taking a carbide scriber – a simple tool that looks like a pen – and dragging its carbide point across one of the facets, the point will scratch through the coating. It will not scratch the diamond, just the coating. Then the “tell-tale” scratch can be seen immediately when one looks at the facet with magnification (a 10-power loupe – the little eye magnifier used by professionals -- will reveal it quickly).


How To Use The Carbide Scriber

A carbide scriber can be used by anyone. No gemological training is required, and it’s quick and easy. Here’s how to do it:

  1. You must make sure you are testing a diamond and not a diamond imitation such as CZ (use an electronic diamond tester).
  2.  The scriber must be a carbide scriber.* There are several different types of “scriber” available for marking various materials, such as stainless steel scribers, but only a carbide scriber will scratch through the surface coatings being applied to diamonds.
  3. Insert carbide point into the scriber (instructions are included with the scriber). Be careful when handling the scriber and avoid dropping it; carbide is very hard, but it is also brittle and the point can break if you drop it and it lands point-first on a hard surface.
  4. Hold the diamond, or piece of jewelry, in a way to ensure a firm grip and then drag the carbide point across a facet with reasonable pressure. Start with a facet on the underside of the diamond (called the pavilion).  
  5. Examine the facet with magnification. Does it appear to have a scratch where you just dragged the scriber across the surface? If so, the stone is coated.
  6. If you see no scratch on the facet on the underside, repeat the process on the top of the stone, pulling the scriber across the big flat facet at the very center of the diamond (called the table facet). If there is a scratch, the stone is coated.
  7. Where colored diamonds are concerned, if there is no scratch on the pavilion or table, it is not surface-coated.
  8. If testing a colorless diamond, if you have not seen any scratch on either the pavilion or table, examine the edge of the stone with magnification (the girdle), being sure to examine the entire girdle area, using a loupe or microscope, to see if there are any characteristics that look like “brush stroke” marks. If there are no brush strokes, the diamond is not coated; if there are brush strokes, this is a diamond treated by a very early, simply type of coating which is a substance that is applied with a small brush.


CAUTION: Do not use a carbide scriber on any gemstone except diamond. The carbide point is extremely hard and will scratch all other gemstones.

* Professional quality carbide scribers are available from the Accredited Gemologists Association at or call AGA headquarters at 619 501-5444.  The cost is $20 for a high quality scriber with interchangeable points. Includes 2 carbide points—one “fine” and one “standard”—which are easily stored within the scriber. Replacement points are also available.