Spinel and garnet are one of the last holdouts in the modern-day growth of gemstone treatments. Besides fracture filling, these two groups of gems typically do not benefit from any standard processes of gemstone enhancement, meaning most of the garnet and spinel in the gem and jewelry market have natural, untreated color. Nonetheless, with the rising popularity of these stones and the resulting increase in their price in recent years, there are certain garnet and spinel treatments that the gem trade should be aware of. CLARITY ENHANCEMENT The most common treatment encountered for both garnet and spinel is one of the most ubiquitous treatments in the gem and jewelry industry—clarity enhancement. Introduction of a foreign substance into the fractures of a gem can hide those fractures, often significantly improving clarity. While this treatment is most commonly associated with emerald, the clarity of any stone with fractures can be improved by filling those fractures. Most commonly oil is used (Figure 1) due to its low viscosity and its ability to easily flow into fractures; however, resins can be used to fill fractures in spinel and garnet as well (Figure 2). Oils and resins can significantly improve the clarity of a spinel or garnet despite the large difference in their refractive indices (Hughes 2020). Identifying this treatment is done largely by microscopic observations. Filled fractures may show gas bubbles trapped when the filler material was introduced (Figure 2). A hotpoint can also be used with the microscope to check for sweating of the filler material when the hotpoint is gently applied to the surface of the stone near the filled fracture. A hand- held longwave UV torch may help as well given that many oils and resins used will fluoresce. If a spinel or garnet is heavily clarity enhanced with oil or resin, Fourier Transform Infrared Spectroscopy (FTIR) may help in identifying the organic compounds used to fill the fractures. A 1.17 ct red spinel from Tanzania before (left) and after (right) heat treatment. The color in this case was negatively affected by the heat treatment. ©GIA. SPINEL HEAT TREATMENT Rumors began circulating through the gem trade that heat treatment was being applied to red and pink spinel from Tanzania. Initial heat treatment experiments generally showed a worsening of the appearance after treatment (Figure 3), although clarity could be significantly improved in some very cloudy material (Saeseaw et al. 2009). Nonetheless, routine testing in the lab at the Gemological Institute of America occasionally identifies a very small number of natural red or pink spinel that has undergone heat treatment. Occasionally, even large spinel with fine color and clarity has been identified as heated such as the 17.02 ct spinel in Figure 4. Given that a significant and consistent improvement in a stone’s appearance has not been demonstrated, it is not obvious why red and pink spinel is being heated at all. In many cases it may simply be that a red spinel is mistakenly identified as a corundum and then heated as if it were a corundum. Alternatively, it may be that a spinel with an undesirable color has been heated in a... A cobalt-diffused natural spinel. The color can diffuse deep enough into the stone to avoid the creation of color concentrations along facet junctions as seen in titanium lattice diffused sapphire. ©GIA.
Spinel and garnet are one of the last holdouts in the modern-day growth of gemstone treatments. Besides fracture filling, these two groups of gems typically do not benefit from any standard processes of gemstone enhancement, meaning most of the garnet and spinel in the gem and jewelry market have natural, untreated color. Nonetheless, with the rising popularity of these stones and the resulting increase in their price in recent years, there are certain garnet and spinel treatments that the gem trade should be aware of. CLARITY ENHANCEMENT The most common treatment encountered for both garnet and spinel is one of the most ubiquitous treatments in the gem and jewelry industry—clarity enhancement. Introduction of a foreign substance into the fractures of a gem can hide those fractures, often significantly improving clarity. While this treatment is most commonly associated with emerald, the clarity of any stone with fractures can be improved by filling those fractures. Most commonly oil is used (Figure 1) due to its low viscosity and its ability to easily flow into fractures; however, resins can be used to fill fractures in spinel and garnet as well (Figure 2). Oils and resins can significantly improve the clarity of a spinel or garnet despite the large difference in their refractive indices (Hughes 2020). Identifying this treatment is done largely by microscopic observations. Filled fractures may show gas bubbles trapped when the filler material was introduced (Figure 2). A hotpoint can also be used with the microscope to check for sweating of the filler material when the hotpoint is gently applied to the surface of the stone near the filled fracture. A hand- held longwave UV torch may help as well given that many oils and resins used will fluoresce. If a spinel or garnet is heavily clarity enhanced with oil or resin, Fourier Transform Infrared Spectroscopy (FTIR) may help in identifying the organic compounds used to fill the fractures. A 1.17 ct red spinel from Tanzania before (left) and after (right) heat treatment. The color in this case was negatively affected by the heat treatment. ©GIA. SPINEL HEAT TREATMENT Rumors began circulating through the gem trade that heat treatment was being applied to red and pink spinel from Tanzania. Initial heat treatment experiments generally showed a worsening of the appearance after treatment (Figure 3), although clarity could be significantly improved in some very cloudy material (Saeseaw et al. 2009). Nonetheless, routine testing in the lab at the Gemological Institute of America occasionally identifies a very small number of natural red or pink spinel that has undergone heat treatment. Occasionally, even large spinel with fine color and clarity has been identified as heated such as the 17.02 ct spinel in Figure 4. Given that a significant and consistent improvement in a stone’s appearance has not been demonstrated, it is not obvious why red and pink spinel is being heated at all. In many cases it may simply be that a red spinel is mistakenly identified as a corundum and then heated as if it were a corundum. Alternatively, it may be that a spinel with an undesirable color has been heated in a... A cobalt-diffused natural spinel. The color can diffuse deep enough into the stone to avoid the creation of color concentrations along facet junctions as seen in titanium lattice diffused sapphire. ©GIA.

Treatments Applied to Garnet and Spinel

Posted on July 1, 2022 by Aaron Palke

Spinel and garnet are one of the last holdouts in the modern-day growth of gemstone treatments. Besides fracture filling, these two groups of gems typically do not benefit from any standard processes of gemstone enhancement, meaning most of the garnet and spinel in the gem and jewelry market have natural, untreated color. Nonetheless, with the rising popularity of these stones and the resulting increase in their price in recent years, there are certain garnet and spinel treatments that the gem trade should be aware of. CLARITY ENHANCEMENT The most common treatment encountered for both garnet and spinel is one of the most ubiquitous treatments in the gem and jewelry industry—clarity enhancement. Introduction of a foreign substance into the fractures of a gem can hide those fractures, often significantly improving clarity. While this treatment is most commonly associated with emerald, the clarity of any stone with fractures can be improved by filling those fractures. Most commonly oil is used (Figure 1) due to its low viscosity and its ability to easily flow into fractures; however, resins can be used to fill fractures in spinel and garnet as well (Figure 2). Oils and resins can significantly improve the clarity of a spinel or garnet despite the large difference in their refractive indices (Hughes 2020). Identifying this treatment is done largely by microscopic observations. Filled fractures may show gas bubbles trapped when the filler material was introduced (Figure 2). A hotpoint can also be used with the microscope to check for sweating of the filler material when the hotpoint is gently applied to the surface of the stone near the filled fracture. A hand- held longwave UV torch may help as well given that many oils and resins used will fluoresce. If a spinel or garnet is heavily clarity enhanced with oil or resin, Fourier Transform Infrared Spectroscopy (FTIR) may help in identifying the organic compounds used to fill the fractures. A 1.17 ct red spinel from Tanzania before (left) and after (right) heat treatment. The color in this case was negatively affected by the heat treatment. ©GIA. SPINEL HEAT TREATMENT Rumors began circulating through the gem trade that heat treatment was being applied to red and pink spinel from Tanzania. Initial heat treatment experiments generally showed a worsening of the appearance after treatment (Figure 3), although clarity could be significantly improved in some very cloudy material (Saeseaw et al. 2009). Nonetheless, routine testing in the lab at the Gemological Institute of America occasionally identifies a very small number of natural red or pink spinel that has undergone heat treatment. Occasionally, even large spinel with fine color and clarity has been identified as heated such as the 17.02 ct spinel in Figure 4. Given that a significant and consistent improvement in a stone’s appearance has not been demonstrated, it is not obvious why red and pink spinel is being heated at all. In many cases it may simply be that a red spinel is mistakenly identified as a corundum and then heated as if it were a corundum. Alternatively, it may be that a spinel with an undesirable color has been heated in a... A cobalt-diffused natural spinel. The color can diffuse deep enough into the stone to avoid the creation of color concentrations along facet junctions as seen in titanium lattice diffused sapphire. ©GIA.

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