How To Test For Diffusion Treated Gems: A Step-By-Step Guide

How To Test For Diffusion Treated Gems A Step-By-Step GuideIf you’ve ever dyed your hair, you know what it’s like to be asked about your natural color. Don’t you wish we could ask gemstones the same thing? Well, since they can’t answer, we have to find out ourselves. How? By performing tests to distinguish the untreated gems from the treated ones. But what exactly does “treated” mean? Keep reading as we answer all your questions about gem treatments, and how to test for diffusion treated gems.

What is Diffusion Testing?

Gemstones are often treated, or enhanced, with heat and/or chemical processes to change their appearance. Treatments generally improve the color and clarity of a stone.

The most common technique is basic heat treatment, a practice that dates back centuries. If heat alone doesn’t do the trick, though, diffusion is the next step.

Diffusion treatment involves letting a certain chemical seep into a gemstone’s surface under extreme heat. The process lasts a long time, usually over a week. The result? Richer color, or a new color entirely, but only on the thin surface layer. Currently, diffusion treatment is most prevalent with corundum gemstones like sapphires and rubies, as well as feldspar, and topaz.

Why Test For Diffusion Treated Gems?

The obvious benefit of testing gems for diffusion is knowing whether your stone’s color is natural or not. While thousands of stones are mined all over the world, the ones with naturally vibrant color and clarity are rare, giving them a higher value.

However, the more important benefit of testing is knowing if the color treatment will last. Diffusion treatment only affects the surface layer, so the color might come off over time with routine repair, cleaning, or polishing. In addition, other treatments like fracture filling or flux healing can make the stone prone to breakage.

So, you know why it works, ready to learn how to test for diffusion treated gems?

Magnification: The Visual Test

One basic method to see if a gem has been diffusion-treated is to simply get a closer look. It’s important to note that magnification alone won’t give you a straight answer on whether the stone is treated. However, it can get you on the right track by revealing certain properties of treated gems.

The key indicator of diffusion is color bleeding — dark concentrations of the stone’s color — around pits and fractures that extend to the stone’s surface. Other indicators of treatment include melted crystals, spotty stripes of color, fractures around inclusions, and pits on the surface. These only indicate heat, though, so keep that in mind.

If you found some clues, you’ll need to examine further. The best way to do that is immersion.

The Immersion Method

An immersion test is the most effective way to check for diffusion treatment. Overall, the test involves immersing the gem in fluid and using a diffused, transmitted light source to see how it looks. Let’s break this process down so you know everything you need.

What You’ll Need

  • Light source  - Generally, a microscope’s light is best for this, but you can also use a polariscope or flashlight.

  • Frosted glass plate or tissue - A frosted glass plate is ideal, but a tissue works just as well so long as it diffuses the light.

  • Clear glass container - If you don’t have an immersion cell, a small beaker or any transparent glass container will work.

  • Tweezers or tongs - You’ll use these to place the stone in the fluid, so choose one that’ll grip the stone and reach your fluid safely.

  • Gloves - Keep yourself safe with these, as some of the fluids, like methylene iodide, can be toxic.

  • Fluid - The ideal fluid has a close refractive index to the stone you’re testing.

The most common immersion fluids are methylene iodide and glycerine. Methylene iodide has a refractive index (R.I.) of 1.75, extremely close to corundum’s RI. Glycerine has an R.I. of 1.47, putting it closer to rubies.

Since an untreated stone will become almost invisible in a fluid with the same R.I., it’s best to choose the fluid that’s closest to the stone you’re testing.

How to Do An Immersion Test


Clear a space and turn on your light source.Place the frosted glass plate (or tissue) over the light.Place your glass container on top.With your gloves on, put a small amount of fluid in the container.Using tweezers or tongs, place the gemstone into the liquid table-down.Observe without magnification.

What to Look For

As we mentioned, untreated gems will seem almost invisible in the fluid. Diffusion-treated gems, on the other hand, will show clear signs of treatments.

The first indicator is color concentration around the edge, or girdle. Uneven diffusion can also result in other areas with patchy coloring, too, such as facet junctions. In fact, facet junctions shouldn’t be very visible at all with an untreated gem, so visible ones could be due to erosion from the diffusion process. If you’re having trouble seeing the facets, try dimming your light or putting more tissue paper over it. This could save you from wrongly deciding the stone is untreated just because the brightness is washing it out.

If you finished testing and found that your gemstone is diffusion treated, you might be wondering: what now?

Should You Keep A Diffusion Treated Gem?

It’s entirely up to you and depends on what qualities you value in a gemstone.

Why Get Rid Of It?

If you’re a purist who values gems for their natural beauty and rarity, you might not care for treatments. You also may not want one if you prefer stones with subtle coloring, rather than rich saturation.

Why Keep The Gem?

There are a few reasons to consider getting a treated gem. For one, the color on some of them often doesn’t occur in nature, making treatment the only way to get it. Treated gems are also much more affordable. Most dealers will offer treated gems at lower prices than their untreated counterparts, even if the stones look the same.

The biggest deciding factor is durability. As previously mentioned, diffusion treatments that only penetrate the stone’s surface might come off with routine maintenance over time. Some diffusing agents, like beryllium, are able to penetrate deeper into the stone. These treatments are less likely to wear down.

In Summary

When diffusion treatment first emerged, gemologists had to wise up in order to identify treatments. Now, thanks to their research, anyone can use these methods to figure out just how natural a gemstone is. Knowing how to test for diffusion treated gems is a skill that helps you choose the right gemstones based on what you value most.

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