Optical Properties of Natural Gemstones and Diamonds - How to Evaluate Them

Diamonds, which captivate the eyes of many people, are either natural or synthetic. Natural diamonds are mined from the earth and synthetic diamonds are artificially made. In recent years, it has become extremely difficult to judge whether a diamond is of natural or synthetic origin by using conventional gemstone appraisal tools and equipment, raising the need for new judgment methods. One such new method utilizes a spectrophotometer.

This article introduces measurements of the reflectance and transmittance spectra of various natural gemstones including diamonds using UV-visible spectrophotometry and Fourier transform infrared spectrophotometry.

Measuring UV-Vis Reflectance Spectra of Diamonds and Other Natural Gemstones

Figure 1 lists the two varieties of diamonds, as well as the natural gemstones, of which spectra were measured. In Figure 2, a sample set in the sample compartment of the SolidSpecTM - 3700DUV ("TM" symbol left out from this point) can be observed.

Diamonds and Natural Gemstones that were Measured.

Figure 1. Diamonds and Natural Gemstones that were Measured.

Inside the Sample Compartment for Measuring a Diamond.

Figure 2. Inside the Sample Compartment for Measuring a Diamond.

The ring section of the diamond was fitted into a widely available sponge substance and then set on the sample base in order to fix the diamond. The light beam aperture mask accessory was then employed to set the diameter of the beam to 1 mm, before measuring the area demonstrated in Figure 1.

The relative diffuse reflectance measurement findings, which do not include specular reflected light, can be seen in Figure 3. These measurements were gathered under the conditions outlined in Table 1. While the spectrum from diamond A does not display any significant peaks, the spectrum from diamond B indicates a peak at 415.2 nm.

Table 1. Measurement Conditions

. .
Instrument Used : SolidSpec-3700DUV
Wavelength Range : 400 to 500 nm
Scan Speed : Very slow
Sampling Interval : 0.2 nm
Slit Width : 1.0 nm

 

The visual features of diamonds vary according to impurity levels (nitrogen molecules) present within. The peak at 415.2 nm, as seen in Figure 3, is thought to show the N3 center, which encompasses three adjacent nitrogen atoms.[1] As the N3 center is widely held to be proof of the natural origin of a diamond, it can therefore be presumed that diamond A is of a synthetic type, while diamond B is a natural diamond.

Reflectance Spectra of Diamonds Blue: Diamond A (Synthetic), Red: Diamond B (Natural)

Figure 3. Reflectance Spectra of Diamonds
Blue: Diamond A (Synthetic),
Red: Diamond B (Natural)

Following this, the transmittance or reflectance spectra of the four natural gemstones in Figure 1 were measured. For ruby, measurements were taken from the reflectance spectrum, while for quartz (rock crystal and amethyst) and fluorite, transmittance spectra were measured, as a result of their high transmittance. The measurement conditions are detailed in Table 2.

Table 2. Measurement Conditions

. .
Instrument Used : SolidSpec-3700DUV
Wavelength Range : 300 to 800 nm
Scan Speed : Medium speed
Sampling Interval : 1.0 nm
Slit Width : 5.0 nm

 

The color and variety of the natural gemstones have an effect on the spectrum shapes obtained. The color values determined from the spectra in Figure 4 are listed in Table 3. Figure 5 illustrates the outcome of the use of the LabSolutionsTM UV-Vis color software to draw these color values, and shows that the values measured do, in fact, align with the true colors of these natural gemstones.

Table 3. Color Values of Each Sample

Sample x y
Rock crystal 0.314 0.330
Amethyst 0.314 0.316
Fluorite 0.314 0.351
Ruby 0.345 0.308

 

Transmittance or Reflectance Spectra of Natural Gemstones Blue: Rock Crystal (%T), Green: Fluorite (%T), Purple: Amethyst (%T), Red: Ruby (%R)

Figure 4. Transmittance or Reflectance Spectra of Natural Gemstones
Blue: Rock Crystal (%T),
Green: Fluorite (%T),
Purple: Amethyst (%T),
Red: Ruby (%R)

Calculated Color Values of Various Natural Gemstones Blue: Rock Crystal, Purple: Amethyst, Green: Fluorite, Red: Ruby

Figure 5. Calculated Color Values of Various Natural Gemstones
Blue: Rock Crystal,
Purple: Amethyst,
Green: Fluorite,
Red: Ruby

Measurement of the Infrared Spectra of Natural Gemstones

The EZClip-13 transmittance measurement attachment, as illustrated in Figure 6, was employed to determine the transmittance spectra of rock crystal and amethyst. The measurement settings are listed in Table 4, while Figure 7 displays the obtained spectra.

Table 4. Measurement Conditions

. .
Instrument Used : IRSpiritTM-T (KBr window)
Optical resolution : 4 cm-1
Accumulation Times : 20
Apodization Function : Happ-Genzel
Detector : DLATGS

 

Sample Set on EZClip-13

Figure 6. Sample Set on EZClip-13

In Figure 7, the shapes of the peaks vary between the obtained spectra at approximately 3,400 cm-1. Amethyst, which has a purple color due to the impurities in quartz, is described as having a wide and robust characteristic peak at approximately 3,435 cm-1.[2] It can be seen through these spectra that the existence of impurities in quartz can be detected by measuring infrared spectra.

Infrared Spectra of Quartz Black: Rock Crystal, Red: Amethyst

Figure 7. Infrared Spectra of Quartz
Black: Rock Crystal,
Red: Amethyst

Summary

UV-visible spectrophotometry and Fourier transform infrared spectrophotometry were employed to determine the reflectance and transmittance spectra of numerous natural gemstones, including diamonds.

Measurements in the UV-Vis range allowed testers to determine whether a diamond was of natural or synthetic origins, while measurements in the infrared range enabled the detection of impurities in quartz, which lead to coloration.

References and Further Reading

[1] Hiroshi Kitawaki, Characterization of natural and synthetic gem minerals and their applied mineralogy: with special references to compositions, physical properties and fine textures in diamond, 2012

[2] Shoji Kurata, Nondestructive Identification of Synthetic Quartz Crystal Seals Using Spectroscopy https://www.jstage.jst.go.jp/article/jafst/11/2/11_2_205/_pdf

This information has been sourced, reviewed and adapted from materials provided by Shimadzu Scientific Instruments.

For more information on this source, please visit Shimadzu Scientific Instruments.

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