Is It Natural or Man-made? Detecting Treated Gemstones with FT-IR Spectroscopy

Rubies (July birthstone) and sapphires (September birthstone) are among the most prized colored gemstones. These gems occur in metamorphic rock, which is formed by intense underground heat or pressure far beneath the Earth’s surface.

Mining for precious colored gemstones is challenging because the deposits are few and usually consist of small quantities of gems scattered throughout a large amount of rock. With deposits too small to be profitable for major mine outfits, colored gemstone mining is often left to small, independent miners who work without the advantages of modern technologies. Precious gemstones are valued according to color, cut, clarity, weight, and in the case of rubies and sapphires, country of origin. In the United States, most colored gems are imported.

According to the U.S. Geological Survey, the market demand for natural, non-diamond gemstones was estimated to be about $1.5 billion in 2013, a trend that is expected to continue. Very small quantities of a variety of gemstones including rubies, sapphires, and emeralds are produced domestically in Arizona, North Carolina, Oregon, California, Utah, Tennessee, Montana, Colorado, Arkansas, and Idaho.

Gemstone Impostors

Synthetic gemstones are created in a lab but have the same chemical and physical properties as real gemstones. Simulants resemble gemstones but have different chemical and physical properties. Some examples are spinel, a simulant used to imitate rubies, sapphires and other colored gemstones, and natural colorless quartz, which can be made to look like rubies or sapphires through “quench crackling” followed by dying. Quench crackled stones can be easily identified, but oftentimes more sophisticated technology is needed.

Ruby vs. Sapphire: What’s the Difference?

Rubies and sapphires are both gem varieties of the corundum group and have the same chemical structure (aluminum oxide). FT-IR spectroscopy can confirm that a stone is corundum and not a simulant, however, the infrared spectra of ruby and sapphire are virtually identical. In fact, synthetic sapphire is colorless and is often used as a window material in near infrared and visible optics because of its unique spectral properties.

Today, red stones are usually classified as ruby, while the other remaining colors are classified as sapphires even though the name “sapphire” was originally reserved for blue stones. The color differences are created by the presence of trace amounts of metal ions that cannot be detected by infrared spectroscopy. For ruby, the red color comes from chrome and iron. For sapphire, the blue color comes from titanium and iron. Pink, green, and yellow stones contain different states of chrome or iron. As with many natural materials, corundum is not particularly rare and colored forms of low-quality stones are readily available. To the contrary, high-quality natural gemstones are rare and very valuable, particularly when the stone has a non-traditional color.

A Solution for Gemological Labs

Heat treatment is often performed on rubies and sapphires to improve color and clarity. A beryllium treatment process greatly enhances the colors of poor quality stones. This high-temperature treatment diffuses beryllium atoms into the corundum crystal and greatly improves the appearance of the stone. Most natural rubies and sapphires have a small peak in the infrared spectrum near 3310 cm-¹ that corresponds to the O-H stretching mode of water. While FT-IR spectroscopy cannot detect the presence of the beryllium atoms, the high temperature of the treatment usually eliminates the water that is trapped in most natural stones and consequently, this peak loss can be measured.

While FT-IR can rapidly detect materials that are “not normal”, which are frequently treated or synthetic, there are always some natural stones that fall within this area due to the huge variance in the chemical makeup of natural minerals. Depending on the potential value of the stone, this rapid screen may be used to identify the need for more extensive testing. In many cases, a full FT-IR analysis can be performed in less than a minute, saving gemologists precious time.

To learn how to verify a sample is an untreated natural ruby or sapphire using FT-IR spectroscopy, read the application note.

EDITOR’S NOTE:   We received a comment from a reader who is a professional gemologist: 

Lab-created (synthetic) corundum is not only colorless, but is made in every color. 

The author’s clarification:

In the case of synthetic corundum, it was developed as a colorless very hard window material. However, she is correct that synthetic gemstones have been created by doping the “pure” colorless corundum with metal ions to create color. I use the term corundum to describe the pure aluminum oxide and sapphire to describe the gemstones.