Microwave Spectroscopy as a Potential Tool for Color Grading Diamonds

At a Glance

Metadata	Details
Publication Date	2021-06-12
Journal	Energies
Authors	Yossi Rabinowitz, Ariel Etinger, Asher Yahalom, Haim Cohen, Yosef Pinhasi
Institutions	Ariel University, Ben-Gurion University of the Negev
Citations	5
Analysis	Full AI Review Included

Executive Summary

This research validates Microwave (MW) spectroscopy as a reliable, non-destructive tool for determining diamond color grade by quantifying nitrogen (N) contamination.

Core Value Proposition: MW spectroscopy provides a robust alternative to traditional UV-visible and Infrared (IR) methods, offering potential cost savings and improved accuracy, especially for rough diamonds where scattered light is problematic.
Nitrogen Correlation: A strong, linear correlation was established between the diamond’s nitrogen concentration (ranging from 0 to 2960 ppm) and its electromagnetic properties in the MW range.
Transmission Method Success: Transmission measurements (S12 parameter) showed clear discrimination between color grades (2a, D, H, L1) at high frequencies, notably 17.72 GHz and 26.025 GHz.
Resonator Method Success: Using a WR187 waveguide as a resonator (3.15-5.85 GHz), the spectral peak location was found to correlate directly with N concentration, offering high reliability and minimal overlap between the lowest (2a) and highest (L1) N samples.
Wavelength Advantage: The long wavelengths used in MW spectroscopy (3.95-26.5 GHz) minimize scattered light effects (which are proportional to 1/λ⁴), making the technique suitable for both rough and polished stones.
Validation: MW results were consistent with N concentrations calculated using established IR absorption spectroscopy (based on the 1095 cm^-1 peak).

Technical Specifications

Parameter	Value	Unit	Context
Total MW Frequency Range Tested	3.95 to 26.5	GHz	Covered by four waveguides (WR187, WR90, WR62, WR42)
Key Transmission Frequency 1	17.72	GHz	Demonstrated linear correlation between S12 and N concentration
Key Transmission Frequency 2	26.025	GHz	Demonstrated linear correlation between S12 and N concentration
Key Resonator Frequency Range	3.15 to 5.85	GHz	Used WR187 waveguide with mirrors
Nitrogen Concentration (Type 2a)	0	ppm	D color grade (lowest N)
Nitrogen Concentration (Type L1)	2960	ppm	L color grade (highest N)
Diamond Mass	0.30 ± 0.02	carat	Master diamonds used for testing
Diamond Permittivity (ε_r)	5.5 to 10	N/A	Relative permittivity in the MW range
Container Material Permittivity	~2.2	N/A	Polyethylene used to fix diamond position
IR Absorption Peak for N	1095	cm^-1	Used for baseline N concentration calculation
WR62 Waveguide Range	12.4 to 18	GHz	Dimensions: 7.8994 mm x 5.7988 mm
WR42 Waveguide Range	18 to 26.5	GHz	Dimensions: 10.668 mm x 4.318 mm

Key Methodologies

The characterization of diamond electromagnetic properties was performed using two primary MW techniques (Transmission/Reflection and Resonant) validated against established IR spectroscopy.

Sample Selection and Preparation:
- Four master diamonds (2a, D, H, L1) of similar size (0.30 ± 0.02 carat, round brilliant cut) were selected, representing varying nitrogen concentrations and color grades.
- Diamonds were fixed in custom polyethylene containers (ε_r ~ 2.2) to ensure repeatable position and orientation within the waveguides.
Baseline Nitrogen Quantification (IR Spectroscopy):
- Infrared spectra were measured using a Bruker ALPHA II Fourier-transform infrared (FT-IR) spectrometer.
- Nitrogen concentration (ppm) was calculated based on the absorption values at the 1095 cm^-1 peak, serving as the ground truth for correlation.
MW Transmission/Reflection Method:
- Diamonds were inserted into four different rectangular waveguides (WR187, WR90, WR62, WR42) covering the frequency range of 3.95-26.5 GHz.
- A Keysight N5230 Vector Network Analyzer (VNA) was used to measure the complex scattering parameters (S-parameters).
- The transmission magnitude (S12 in dB) was measured 20 times for each diamond type at various frequencies (e.g., 7.0375 GHz, 17.72 GHz, 26.025 GHz).
MW Resonant Method:
- The WR187 waveguide (3.95-5.85 GHz) was converted into a resonator by placing mirrors at both ends, increasing the effect of the diamond’s electromagnetic properties via multiple reflections.
- The VNA measured the S12 parameter, focusing on the precise frequency location of the spectral peaks (resonances).
Statistical Analysis:
- Statistical distributions (bell-shaped graphs) and standard deviations (STD) were generated from the 20 repeated S12 measurements for each diamond type to assess the reliability and overlap between color grades.

Commercial Applications

This MW spectroscopy technique provides a foundation for developing advanced, non-destructive diamond characterization tools for the gemological and materials industries.

Gemological Laboratories and Trade:
- Objective Color Grading: Replacing subjective human grading or expensive optical equipment with a quantitative, objective electromagnetic measurement.
- Rough Stone Evaluation: Determining the nitrogen content and potential color grade of rough diamonds before the costly polishing process, maximizing yield and valuation.
- Cost Reduction: MW detectors and sources are generally much cheaper than high-end FTIR units, potentially lowering the barrier to entry for advanced grading technology.
Advanced Materials Manufacturing (CVD/HPHT Diamonds):
- Quality Control: Rapid, non-destructive assessment of impurity levels (specifically nitrogen) in synthetic diamonds used for high-performance applications (e.g., thermal management, high-power electronics).
- Dielectric Characterization: Precise measurement of the effective permittivity (ε_r) of diamond materials, critical for applications in high-frequency electronics and microwave windows.
Instrumentation and Sensor Development:
- Novel Spectroscopy Tools: Development of specialized waveguide and resonator systems optimized for characterizing low-loss, high-permittivity dielectric materials.

View Original Abstract

A diamond’s color grading is a dominant property that determines its market value. Its color quality is dependent on the light transmittance through the diamond and is largely influenced by nitrogen contamination, which induces a yellow/brown tint within the diamond, as well as by structural defects in the crystal (in rare cases boron contamination results in a blue tint). Generally, spectroscopic instrumentation (in the infrared or UV-visible spectral range) is used in industry to measure polished and rough diamonds, but the results are not accurate enough for precise determination of color grade. Thus, new methods should be developed to determine the color grade of diamonds at longer wavelengths, such as microwave (MV). No difference exists between rough and polished diamonds regarding stray light when the MW frequency is used. Thus, several waveguides that cover a frequency range of 3.95-26.5 GHz, as well as suitable resonator mirrors, have been developed using transmission/reflection and resonator methods. A good correlation between the S12 parameter and the nitrogen contamination content was found using the transmission/reflection method. It was concluded that electromagnetic property measurements of diamonds in the MW frequency range can be used to determine their nitrogen content and color grading. The MW technique results were in good agreement with those obtained from the infrared spectra of diamonds.

Tech Support

Original Source

DOI: https://doi.org/10.3390/en14123507

References

2020 - Natural-color D-to-Z Diamonds: A Crystal-clear perspective [Crossref]
2018 - The effect of blue fluorescence on the colour appearance of round-brilliant-cut diamonds [Crossref]
2020 - Naturally colored yellow and orange gem diamonds: The nitrogen factor [Crossref]
2018 - Natural-color blue, gray, and violet diamonds: Allure of the deep [Crossref]
2018 - Natural-color pink, purple, red, and brown diamonds: Band of many colors
2019 - Natural-color fancy white and fancy black diamonds: Where color and clarity converge
2005 - Unusually large novelty cut [Crossref]