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6CCVD Research

Chromaticity Study of Yellow HTHP Lab-Grown Diamonds Based on Spectroscopy

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2025-10-31
JournalCrystals
AuthorsZ. Y. Peng, Youping Sun, Mingming Xie, Zheng Zhang, Bin Meng
InstitutionsKunming University of Science and Technology
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This research provides a quantitative spectroscopic and colorimetric analysis of yellow High-Temperature High-Pressure (HTHP) lab-grown diamonds, establishing objective metrics for color classification.

  • Quantitative Color Correlation: A strong positive linear correlation (R2 = 0.90) was established between the Nitrogen Content (Nc) and the Yellowness Index (YI E313), defined by the equation: Nc (ppm) = 0.17 * YI E313 + 124.40.
  • Defect Confirmation: Samples were confirmed as Type Ib diamonds (isolated nitrogen, C aggregate) synthesized via HTHP from graphite, followed by post-growth irradiation/treatment, evidenced by characteristic NV0 (575 nm) and NV- (637 nm) peaks and the absence of the [Si-V]- defect (737 nm).
  • NV- Role in Color: The concentration ratio of the NV- defect (R) is positively correlated with both Nc and YI E313. R ranges from 0.31 to 0.80, confirming NV- as the primary driver for increasing yellow saturation.
  • Objective Grading System: A new color classification system is proposed based on YI E313: Light (70-80), Intense (80-90), and Deep (90-100), providing a standardized metric for gemological evaluation.
  • Crystal Quality: Raman FWHM analysis showed that samples with deeper color (Y5-Y8) generally exhibited higher crystallinity (lower FWHM), although FWHM correlation with YI E313 was deemed unreliable for precise grading.

Technical Specifications

Section titled “Technical Specifications”
ParameterValue RangeUnitContext
Nitrogen Content (Nc)136.14 - 141.51ppmCalculated using modified IR absorption formula (Eq. 7).
Yellowness Index (YI E313)73.63 - 99.19DimensionlessASTM E313 standard, D65 light source.
Hue Angle (h)1.39 - 1.48RadiansCalculated from CIELAB a* and b* parameters.
NV- Concentration Ratio (R)0.309 - 0.804DimensionlessRatio of NV- peak intensity to total NV center intensity.
Lightness (L*)72.07 - 81.3DimensionlessCIELAB parameter.
Yellow Chromaticity (b*)37.04 - 56.25DimensionlessCIELAB parameter (positive indicates yellow component).
Diamond Intrinsic Raman Peak1332cm-1Primary carbon peak position.
Isolated Nitrogen IR Peaks1130, 1344cm-1Characteristic absorption peaks for Type Ib diamonds.
NV0 Zero-Phonon Line (ZPL)575nmCharacteristic PL peak for neutral NV center.
NV- Zero-Phonon Line (ZPL)637nmCharacteristic PL peak for negatively charged NV center.
Nc vs. YI E313 Correlationy = 0.17x + 124.40Nc (ppm) vs. YI E313Positive linear fit (R2 = 0.90).

Key Methodologies

Section titled “Key Methodologies”

The study utilized a combination of spectroscopic and colorimetric techniques on eight yellow HTHP lab-grown diamond samples (Y1-Y8).

  1. Photoluminescence (PL) Spectroscopy:

    • Instrument: JASCO NRS7500 micro laser Raman spectrometer.
    • Conditions: Liquid nitrogen environment, 532 nm laser source, 560-800 nm range.
    • Analysis: Measured intensities of NV0 (575 nm) and NV- (637 nm) peaks to calculate the concentration ratio (R) of the NV- defect.

  2. Infrared (IR) Spectroscopy:

    • Instrument: TENSOR27 Fourier Transform Infrared Spectrometer (Bruker).
    • Conditions: Reflection method, 400-4000 cm-1 range, 4 cm-1 resolution.
    • Analysis: Confirmed Type Ib classification via 1130 cm-1 and 1344 cm-1 peaks. Nitrogen content (Nc) was calculated using a modified formula referencing the 1130 cm-1 absorption peak intensity normalized against the 2120 cm-1 carbon-nitrogen triple bond peak.

  3. Raman Spectroscopy:

    • Instrument: Renishaw inVia microscope Raman spectrometer.
    • Conditions: 785 nm laser source, 100-2000 cm-1 range.
    • Analysis: Measured the Full Width at Half Maximum (FWHM) of the intrinsic diamond peak (1332 cm-1) to assess crystal quality and detected the graphite ‘D’ peak (1418 cm-1).

  4. Colorimetry Testing (CIELAB):

    • Instrument: FUV-007 UV-Vis-NIR spectrometer.
    • Conditions: Reflection method, D65 light source, 10° observer angle.
    • Analysis: Measured L*, a*, and b* parameters. These were used to calculate the Hue Angle (h) and the Yellowness Index (YI E313) according to ASTM E313.

Commercial Applications

Section titled “Commercial Applications”

This research provides critical data for the manufacturing, quality control, and grading of synthetic diamonds, particularly in high-value markets.

  • Gemological Industry (Grading and Valuation):

    • Establishes a quantitative, objective standard (YI E313 and Nc) for classifying fancy yellow lab-grown diamonds (Light, Intense, Deep), reducing reliance on subjective visual grading.
    • Provides verifiable metrics for certification reports, enhancing consumer trust and market transparency for HTHP products.

  • Advanced Materials Manufacturing (Defect Engineering):

    • The precise correlation between Nc and the NV- ratio (R) is vital for manufacturers aiming to produce diamonds optimized for specific functional applications.
    • Quantum Sensing: Since the NV- state is the active center for quantum applications, controlling the synthesis and post-growth treatment to maximize R (up to 0.80 achieved in this study) is a direct pathway to creating high-performance quantum precursors.

  • HTHP Process Optimization:

    • Allows engineers to fine-tune nitrogen doping levels during HTHP growth to reliably hit specific color targets, maximizing yield and minimizing off-color production.
    • The spectroscopic signatures (e.g., residual Ni-Fe catalyst peaks) serve as quality control checkpoints for the synthesis process.

  • Diamond Type Verification:

    • The combination of Type Ib IR signatures and the absence of the [Si-V]- defect (737 nm) provides a clear spectroscopic fingerprint distinguishing HTHP-synthesized and treated diamonds from CVD-grown or natural diamonds.
View Original Abstract

In recent years, lab-grown diamonds have become more popular in both domestic and international markets for their rich color palette. Research on yellow lab-grown diamonds has primarily focused on spectroscopic and defect characteristics currently, while the study has largely focused on nitrogen content and related color-causing mechanisms, such as NV series defects. However, the relationship between nitrogen content and defects and color is limited. In this study, eight lab-grown diamonds with varying yellow shades were selected as samples to be studied by photoluminescence spectra, infrared spectra, Raman spectra, and colorimetry testing. Based on the colorimetric parameters L*, a*, and b*, the standard formula for the yellowness index, the intensities of the NV0 and NV− peaks in the photoluminescence spectra and the absorptivity in the infrared spectra, the hue angle h, the yellowness index YI E313, the concentration ratio of NV− defect in NV color centers R, and the nitrogen content NC were calculated. Results indicate that characteristic peaks of NV series defects as a specific photoluminescence signature, notably the absence of [Si-V]− defect, demonstrate that the samples are high-temperature, high-pressure diamonds derived from graphite that underwent post-growth irradiation. The specific infrared signature indicates that the type of samples is type Ib, attributed to isolated nitrogen (C aggregate). The intrinsic peak of diamond is detected in Raman spectra, with symmetric stretching vibrations of C and N and the ‘D’ peak of graphite is detected as well. Meanwhile, the yellowness index shows a negative correlation with hue angle, a positive correlation with concentration ratio, and a positive linear correlation with nitrogen content, the equation y = 0.17x + 124.40. The yellowness index is divided into three levels: 70-80, 80-90, and 90-100. The yellow hue of samples is light between 70-80, intense between 80-90, and deep between 90-100.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2004 - Spectral Study of Transition Metal Ions in Colored Diamond
  2. 2022 - Diamond Spectroscopy, defect centers, color, and treatments [Crossref]
  3. 2018 - Research on the Chromogenic Mechanism of Natural, Synthetic, and Processed Blue Diamonds
  4. 2018 - Blue boron-bearing diamonds from Earth’s lower mantle [Crossref]
  5. 2024 - Spectroscopic characterization of rare natural pink diamonds with yellow color zones [Crossref]
  6. 2025 - Spectroscopy of green fluorescence rims on rough pink diamonds from Argyle mine [Crossref]
  7. 2020 - Discussion on the Identification of an Irradiated Green type IIa Diamond
  8. 2022 - Difference of lattice radiation damage and spectroscopic characterization in natural and artificial radiation green diamonds
  9. 2001 - Evolution and colouration of lattice defects in diamonds at high pressure and high temperature

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