# Characterization of synthetic yellow diamonds used in the cutting and polishing industry

Gianfranco Gobbo, Quentin Lachuer, Sophie Depriester, Tommaso

Maurizi-Enrici, Gérard Panczer, Yann Le Godec, Irene Calliari, Sylvie Le

Floch, Vittoria Pischedda

► To cite this version:

Gianfranco Gobbo, Quentin Lachuer, Sophie Depriester, Tommaso Maurizi-Enrici, Gérard Panczer, et al.. Characterization of synthetic yellow diamonds used in the cutting and polishing industry. Diamond and Related Materials, 2025, 158, 10.1016/j.diamond.2025.112656. hal-05233981

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Submitted on 1 Sep 2025

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# Characterization of synthetic yellow diamonds used in the cutting and polishing industry

Gianfranco Gobbo<sup>a</sup>, Quentin Lachuer<sup>b</sup>, Sophie Depriester<sup>b</sup>, Tommaso Maurizi-Enrici<sup>c</sup>, Gérard Panczer<sup>b</sup>, Yann Le Godec<sup>d</sup>, Irene Calliari<sup>a</sup>, Sylvie Le Floch<sup>b, *,</sup> Vittoria Pischedda<sup>b, *</sup>

## ARTICLE INFO

### Keywords:

Synthetic yellow diamonds

Morphology

Spectroscopy

Mechanical testing

Thermal treatment

## ABSTRACT

This study investigates the physico-chemical properties of synthetic yellow diamonds used in the cutting and polishing industry, focusing on four High-Pressure High-Temperature (HPHT) diamond batches with identical average grain sizes but different mechanical properties and industrial applications, sourced from various suppliers. The analysis encompasses their morphology, crystal structure, chemical composition, and thermal and mechanical durability. Techniques such as microscopy, XRF, XRD, SEM, Raman spectroscopy, FTIR, UV analysis, and polariscope measurements were utilized. The findings reveal significant variations in batch quality, largely influenced by the morphological integrity of cube-octahedral crystals. Chemical analysis identified trace elements, such as Fe, Al, and Cr, which affect crystal growth and defect formation during synthesis. Mechanical testing showed that one batch exhibited superior resistance due to its optimal morphology and minimal defects, while two lower-cost batches demonstrated moderate resilience. Thermal treatment results indicated that carbide-forming elements enhance resistance to graphitization, a critical factor for high-temperature applications. Although the findings align with general commercial practices, the justification for pricing discrepancies remains unclear, emphasizing the need for detailed batch characterization in the industry.

## 1. Introduction

Natural diamonds are generated deep within the Earth, precisely in the upper mantle, where carbon atoms are put under conditions of intense pressure and heat, causing the atom of carbon to covalently bond with four others and form a cubic structure with high density and high packing factor. This arrangement of carbon atoms gives diamonds their sought-after properties: extraordinary stiffness, hardness, durability, and transparency as well as their ability to easily conduct heat. Such exceptional properties allow diamonds to be used in several industrial applications like cutting, drilling, polishing, or the roughing of natural stones such as granitic marble.

However, the extraction of natural diamonds is set for a rapid change, mainly due to an important increase in overall costs. Existing mines are lacking resources and new diamond deposits are found deeper underground in less accessible areas [1–4]. Fortunately, technology has helped to artificially replicate the structure of this material since 1950.

The first public announcement of the successful manufacture of synthetic diamond was conducted in 1955 using HPHT (high pressure-high temperature) [3]. Briefly, the HPHT technique consists of placing three main ingredients inside a reaction cell: a 'diamond substrate', which functions as a growth seed; a mixture of metals, which serves as a metallic flux to accelerate the growth of the crystals; and a carbon source made of pure graphite, which initiates the process. Under high temperatures, the metal catalyst melts and graphite dissolves; therefore, free carbon atoms spread into the molten metal and then migrate to the base of the reaction cell. Subjected to high temperatures (900–1300 °C) and high pressures (5–6 GPa), the system reaches supersaturation, and diamonds begin to grow. The process described occurs on top of the seed, which functions as a growth seed, guiding the nucleation of the material [5–10].

Nowadays, more than 56 % of artificial diamonds are produced in China by the HPHT process [2]. The quality and price of synthetic diamonds can vary significantly depending on their industrial application,

<footnote><sup>a</sup> Department of Industrial Engineering, University of Padua, Via Marzolo 9, 35122 Padova, Italy</footnote>

<footnote><sup>b</sup> Institut Lumière Matière, Université Claude Bernard Lyon1, UMR5306-CNRS, 69622 Villeurbanne, France</footnote>

<footnote><sup>c</sup> Abra Iride SpA, Via E. Minato, 28 – 31039 Riese Pio X (TV), Italy</footnote>

<footnote><sup>d</sup> Sorbonne Université, CNRS, MNHN, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), F-75005 Paris, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: sylvie.le-floch@univ-lyon1.fr (S. Le Floch), vittoria.pischedda@univ-lyon1.fr (V. Pischedda).</footnote>
with factors such as size, shape, colour, and the presence of impurities playing crucial roles in determining their suitability and cost. In the cutting and polishing industry diamonds are usually embedded in a metal matrix. The sintering of a metallic composite with diamonds is generally realized at a temperature between 600 and 900 °C, several studies reported a decrease in quality during heating already at 680 °C [11,12].

In industries where large amounts of diamond powder are used as raw material, the quality of the diamonds is crucially dependent on the repeatability and consistency of the purchased batches over time. Therefore, the impact strength value is measured, as it directly relates to the hardness and resiliency of the powders and ensures repeatability. However, the seller may not provide a detailed characterization of the diamond batches, making it difficult to determine the primary factors influencing the mechanical and thermal durability of the diamonds. The mechanical and thermal properties of these diamonds can vary due to a multitude of factors, primarily influenced by their synthesis and processing methods [1].

In the HPHT process, it has been observed that the addition of Si or Ge to Mg-based catalysts increases the growth rate and/or facilitates preferential diamond growth on the seed, resulting in the production of relatively large and well-shaped cuboctahedral diamonds [8,9,13]. On the opposite side, excessive Zn as an additive in conventional metallic flux (such as Fe or Ni) has a suppressive effect on nucleation, resulting in badly crystallized diamonds [14].

Studies carried out on synthetic diamonds usually focus on altering the diamond growth process through the addition of different additives [15], the use of various types of catalysts [5,7,13] or by subjecting the system to different thermodynamic conditions [16]. There are very few studies that focus on the end-products made by private companies for customers. Although these products are commercially available, detailed information about their characteristics is often lacking [17].

In this study, four synthetic diamond batches, each distinguished by varying qualities and sold at different prices, are characterized using different laboratory techniques. We used optical and scanning electron microscope (SEM) to observe surface morphology, a polariscope to analyze stress distribution, X-ray fluorescence (XRF) to determine chemical compositions, X-Ray Diffraction (XRD) to determine the presence of different phases and characterize the structure, and vibrational techniques like Raman spectroscopy and Fourier-transform infrared spectroscopy (FTIR) to analyze eventual doping and substitutions. These characterizations aim to assess the primary parameters influencing the quality of the diamonds and whether they align with their respective prices.

# 2. Method

## 2.1. Materials

The study was carried out on four batches of HPHT micro-diamonds. Three batches, labelled A1, A2, A3 in this paper, belong to the same supplier while the sample labelled B1 is sold from a different supplier (the name of the suppliers are not given because the samples are only taken as examples, giving information's about a particular seller is behind the aim of the paper). Each batch has been selected with the same particle size distributions of 30/40 mesh (the size range would be between 425 and 600 µm) but they vary both in their intended use or quality and their price. A1 is the most expensive of the batches, it is sold as having a controlled cuboctahedral shape and purity, exceptional diaphaneity, high strength and thermal stability. It is claimed to be designed for applications requiring durability under intense conditions and high-load applications, such as mining, drilling and cutting of hard materials like reinforced concrete and stone. A2 is claimed to have a well-defined crystalline structure with minimal impurities and to offer good strength and thermal stability. It is well-suited for applications such as circular saws, gang saws, and core bits that operate under

medium to high loads.

A3 and B1 are the most affordable options and present a combination of various crystalline shapes, resulting in moderate strength and friability. They are widely used in grinding wheels and serve as an economical choice for cutting tools intended for low-medium load applications.

## 2.2. Optical microscopic study

The optical observations were carried out using a Zeiss microscope equipped with a Leica camera EC3 and objectives. Both reflection and transmission modes were employed to characterize colour, crystal shape, surface size, and morphology, as well as to detect visible defects and inclusions. Several pictures (approximately 10 per batch) were captured and analyzed using ImageJ, an image-processing software equipped with various functions. In this study, surface area calculation and particle dimension measurement were used. To calculate the surface area, each diamond was enclosed using the 'polygon' tool. For consistent visible surface distribution, the analysis was performed on a range of 200-400 diamonds for each batch. The values of the surfaces were exported on OriginPro software on which multiple charts are traced to evaluate the number of times a surface is repeated.

## 2.3. X-ray fluorescence measurements

XRF technique is an elementary analysis technique, which allows one to determine the chemical elements within the material. The measurements were conducted with a NITON XL3t 980 GOLDD + Ag-Thermo Scientific™, which does not allow the determination of chemical species lighter than magnesium. The diamonds were put on high-purity polypropylene filmed sample holder that does not absorb X-ray. About twenty diamonds per batch were analyzed and each analysis was done in 120 s using a method called "ore calibration". The calibration is suitable for detecting trace elements in the diamonds, which are often present in very low concentrations.

## 2.4. X-ray diffraction

For XRD we used a Bragg-Brentano diffractometer Bruker D8, at the copper wavelength of 1.5418 Å, at the Centre of Diffractometry Henri Longchambon of the University of Lyon.

The software Match1 was used to identify the phases. As the crystals have size of around 0.5 mm, it is difficult to obtain accurate structural information as large crystals are more prone to preferred orientation effects, this can bias the diffraction pattern and lead to the extinction of certain peaks of diffraction. When searching for candidates on "Match1" it is imperative to account for these potential issues.

## 2.5. Raman spectroscopy

Raman spectroscopy is a versatile tool for characterizing various properties of diamonds, including their structure, composition, defects, residual stress and vibrational properties [18]. This analysis was conducted on a homemade Raman spectrometer using a green 532 nm wavelength laser in a backscattering configuration. The power of the laser on all analyses remained identical to 0.2 mW, with variable acquisition time.

## 2.6. Fourier transform infrared spectroscopy

The FTIR is a vibration-absorption analysis technique, which allows to reveal the vibration modes of C—C in the diamond structure as well as those of the various nitrogen (isolated, aggregates, platelets) and hydrogen incorporated in the diamond lattice. FTIR spectra were recorded using a IS20 Thermo Scientific Nicolet spectrometer as well as a Perkin Elmer EasyDiff diffuse reflectance accessory using 30 scans. In
such configuration, the diamonds deposited on a metal mirror were
directly illuminated by IR beam without going through them and the IR
absorption recorded by diffuse reflectance.

## 2.7. UV

A UV-analysis, utilizing diffuse reflection, was undertaken to evaluate the material's absorbance parameter. The Ocean Optics instrument used is characterized by a lamp, a USB200+ spectrometer, optical fibers, and a sample holder made from visible-range totally-reflective Spectralon®. To ensure accurate calibration and eliminate background noise, initial measurements were conducted without samples, with the lamp both on and off, respectively. This calibration process allows precise subtraction of blank measurements to obtain the absorbance spectrum.

## 2.8. Polariscope

A Krüss polariscope is used to analyze and visualize the optical
properties of materials, particularly those that exhibit birefringence. The
polariscope consists of two crossed-polarizing filters: the polarizer and
the analyzer. The diamonds were placed on a glass support placed be-
tween the two polarizers. The analyzer, placed perpendicular to the
polarizer, allows only light waves polarized in a certain direction to pass
through. If strain interference patterns occur during a full rotation of the
analyzer, it suggests the sample experienced mechanical constraints
during growth. Conversely, if total extinction is observed, it indicates
that the sample remains isotropic and does not have residual stresses.

## 2.9. Friability tester and thermal treatment

Impact strength of the diamond powders was evaluated via Retek
friability tester, following the standard procedure with a minimum of
five tests for the virgin powders and one test for the thermal-treated
powders.

The system consists of a capsule with a suitably shaped inner cavity, where the diamonds are placed together with a tempered steel ball. The capsule is subjected to high-frequency oscillations along the larger axis of the cavity. With each oscillation, the sphere is projected against the bottom walls of the capsule. At every cycle, two impacts occur between the ball and the diamond grains. The mechanical tests for impact strength measure the number of cycles needed to break or to cleave a predetermined number of crystals, determined through statistical equations. Different batches over time must exhibit a statistical match in terms of the number of cycles.

A thermal treatment (TT) at 680 °C/ 24 h in a muffle furnace was also
carried out on A1, A3 and B1 samples in order to evaluate the thermal
stability of the powders.

## 2.10. Scanning electron microscope

SEM images were acquired at the PNNL-ILMTech platform at Uni-
versity of Lyon using a Tescan CLARA instrument, operating at a beam
landing energy of 1 keV with secondary electron detectors.

# 3. Results and discussion

## 3.1. Optical microscopy analysis

### 3.1.1. Morphology and analysis of the distribution of the surface of diamonds

The optical observations revealed that the cube-octahedral shape was notably predominant among the diamonds analyzed (see Fig. 1). This is the typical shape obtained with HPHT synthetic diamonds under pressure conditions of 5–6 GPa and temperature conditions of 1300–1500 °C [19]. By controlling the thermodynamics properties of the process, together with varying additives or catalysts, it is possible to

<figure><img src="image_1.png" /><figcaption>**Fig. 1.** Top: Microscopic images of HPHT diamonds (type Ib), illustrating the variety of shapes observed in the batches: (a) well-formed cuboctahedral diamond, (b) irregular or damaged cuboctahedral diamond, and (c) multiple interpenetrated diamonds. Bottom: The two characteristic orientations of a cuboctahedral diamond.</figcaption></figure>

grow gems with well-defined crystal forms.

To evaluate the uniformity and morphological quality of the di-
amonds, we examined them under a microscope and analyzed the upper
surface of each diamond. In total, we observed 1400 microdiamonds.
Their upper surface areas were automatically measured using ImageJ
software. The images (snapshots) were processed to remove noise and
background interference. To establish a reference for comparison, we
used a theoretical model. This model assumes that perfectly shaped di-
amonds can rest in only two specific orientations, as shown in Fig. 1.
Each of these orientations corresponds to a precise surface area, marked
with purple lines in Fig. 2. Ideally, since all the batches are manufac-
tured around the same size, a sample composed exclusively of well-
shaped microdiamonds would show surface values near the purple
lines, any deviation from these two ideal surface values suggests the
presence of shape irregularities, signs of etching, or diamond size dif-
ferences. The x-axis in Fig. 2 represents the projected surface area (in
mm²) of individual microdiamonds. To statistically represent the vari-
ation from the ideal shapes, we applied a bell curve, which helps visu-
alize how much the measured surfaces differ from the theoretical values.

For samples A1 and A2, the distribution more closely follows a
Gaussian trend around the theoretical model. For samples A3 and B1,
however, we observed a more asymmetric distribution, with larger
extreme values compared to the previous two samples. This simple
analysis allows us to observe that samples A1 and A2 exhibit greater
homogeneity in shape and size compared to samples A3 and B1.

For sample A1, the rate of well-shaped cube-octahedral diamonds varies from 70 % to 75 %, while the rest have a more irregular or spherical shape. Different is the batch A2, where we can observe a rate of cube-octahedral diamonds between 55 % and 60 %. Both samples A3 and B1 have from 45 % to 50 % of cube-octahedral diamonds, and both A3 and B1 display several interpenetrated diamonds and poorly shaped gems.

Given that A1 and A2 are claimed to have superior strength and
thermal stability compared to A3 and B1 batches, the percentage of well-
formed cube-octahedral diamonds, along with shape and size homoge-
neity, can be considered the key parameters for assessing sample quality.
<figure><img src="image_2.png" /><figcaption>**Fig. 2.** The surface area distributions of approximately 400 microdiamonds for each of the batch A and around 200 for batch B were analyzed using ImageJ. The batch A1 is marked in grey, A2 in red, A3 in blue and B1 in green. Additionally, the reference values of the theoretical model are indicated with violet bins.</figcaption></figure>

## 3.2. X-ray fluorescence results

X-ray fluorescence analysis was carried out in order to evaluate the presence of inclusions and trace elements. Fig. 3 shows the charts with the main elements detected for each batch.

Fe, Nb, W, Mo, Y, Cr, Zr, Ni and Cu, at low concentrations (< 3500 wppm) are detected. Fe is detected in each sample by high values of wppm and the corresponding bin in the chart is marked in black. Indeed, batch A2 exhibits the highest value among the batches and during magnetic tests (neodymium magnet), it exhibited a stronger magnetization of diamond grains compared to the other batches.

All samples contain traces of carbide-forming elements (i.e. Nb, W, Mo, Cr, Y, and Zr). Ni and Cu are observed in samples A2 and A3 and marked in yellow. A small amount of Cd is also detected in very low quantities (< 761 wppm) in sample B1. These elements as single elements or compounds (i.e. carbides, oxides and various silicates) have been reported as having a catalytic effect on diamond growth [20]. Moreover, various elements used in the synthesis process have been reported to directly influence key parameters of HPHT synthesis, affecting both the quality and morphology of the resulting diamond powders [5,7,10,20].

Fe-based melts are widely used as “cheap” catalysts for growing synthetic diamonds at HPHT. The catalysts are chosen in order to decrease the melting point, suppress the carbide formation and achieve the targeted outputs of the process. Ni and Co are commonly added to Fe and these systems are widely reported [8,20,21]. Sample A2 and A3 exhibits both Ni and Cu. Cu addition, as Zn addition [14], may be tuned into the Fe-Ni-C system in order to achieve specific properties due to their unconventional behavior as catalysts [14,22]. Both Zn and Cu require higher temperatures compared to other conventional catalysts to initiate the process of growth [14,22,23]. Cu exhibits a slow growth rate in the Cu—C system, which allows for a controlled and high-quality growth of diamond films with fewer defects [22,23]. Zn is reported to improve the growth rate when added to conventional catalysts like Fe—Ni. However, beyond a certain concentration, it suppresses the

nucleation resulting in lower-quality diamonds [14].

Al, detected only in A1 batch and marked in red in Fig. 3, is added to Fe-Ni-C or Fe-Mn-C systems and is reported to significantly reduce spontaneous nucleation. This results in a shift in the crystal size distribution toward a predominance of larger crystals [5,24] and a reduction in defects. Al is also reported to be a nitrogen getter [24–26], added to reduce the nitrogen concentration in diamond growth at HPHT. Indeed, N and other atom inclusions are defects causing lattice distortions [27–29].

The addition of various trace elements such as Nb, W, Mo, Cr and Zr, marked in blue, can be strategically used to form inclusions during synthesis [20], thereby tuning the mechanical properties of the diamond powders. The presence of small contents of Ag, Y, Cd and Ga, marked in green, is only suggested as cannot be univocally attributed. Interestingly, A1, A2, A3 and B1 share some common trace elements, a few of which are present in similar concentrations. The fact that some elements are found in similar concentrations across all the A samples, while one or two elements are missing in certain samples, suggests that these batches, obtained from the same supplier, are likely mixtures made by combining two, three, or more different types of diamond powders. In contrast, sample B1 is considered purer than the A samples. It is likely a blend of only one or two diamond powders, as fewer elements were detected in it.

## 3.3. X-ray diffraction results

The XRD patterns of samples A1, A3 and B1 are reported in Fig. 4. As mentioned in §2.4, the large size of the diamonds resulted in distorted intensity ratios of the diffracted peaks. The peaks at 43, 52, 76, 92 and 96° may correspond to the crystallographic plane (111), (200), (220), (311) and (222) of the diamond with cubic structure and parameter *a* = 3.566(5) Å. For B1 sample, the peak at 92° may also indicate the presence of the high-temperature γ iron phase with cell parameter *a* = 3.569 Å. Given that inclusions in diamonds can experience pressures of several GPa [30], it is plausible that the iron γ phase formed during the growth process could have been preserved. However, with limited statistical
<figure><img src="image_3.png" /><figcaption>**Fig. 3.** Trace-element XRF analysis was conducted on four different batches of HPHT diamonds. Samples A1, A2, and A3 showed a heterogeneous composition with shared elements, while B1, sourced from a different supplier, contained fewer inclusions (measured in wppm on a log scale). Only elements present at concentrations above 5 ppm are reported.</figcaption></figure>

data, interpreting further XRD results is challenging.

### 3.4. Raman spectroscopy results

In Fig. 5 the selected representative Raman spectra for each batch are shown. At lower frequencies, the intense and sharp Raman band at $1331.6 \text{ cm}^{-1}$ characteristic of carbon sp³ hybridization of the cubic crystalline diamond [29,31] can be observed.

The fitting of the full width at half maximum (FWHM) in Table 1 serves as an indicator of the crystalline quality of all diamonds. The FWHM values for all batches hovered around 3.00(3) $\text{cm}^{-1}$ indicating slightly better crystallinity on samples A1 and A2 with respect to A3 and B1.

The bands at 1430, 3135, 3665 and 4111 $\text{cm}^{-1}$ could suggest the presence of nitrogen-related defects. As a matter of fact, additives such as Fe, W or Ni during the manufacturing of synthetic diamonds are real N-traps and testify to the poor quality of diamonds [25,26,31,32]. The most common lattice defects associated with nitrogen in diamond are the $NV^-$ and $NV^0$ defects. These defects consist of a substitutional nitrogen atom N paired with an adjacent lattice vacancy V within the diamond structure. Both $NV^-$ and $NV^0$ defects create electronic states within the diamond's band gap and facilitate the transfer of electrons from the valence band to the conduction band through photon absorption (575 nm for $NV^0$ and 638 nm for $NV^-$) [25,28,33].

The intensity of the peaks is influenced not just by the quantity of $NV^-$ centers, but also by N-related crystal defects. The absence of Raman bands between $1550$ and $1600 \text{ cm}^{-1}$ (which is characteristic of sp² carbon of graphite), marks the absence of graphitization.

### 3.5. FTIR spectroscopy results

In Fig. 6, representative FTIR spectra for the four diamond batches are presented. The bands between $2700$ and $1400 \text{ cm}^{-1}$ (Fig. 6a) correspond to the intrinsic lattice vibrations of diamonds while the peaks between $2950$ and $2700 \text{ cm}^{-1}$ indicate the presence of C—H bonds. Hydrogen is evidenced by small peaks at 2919 and $2849 \text{ cm}^{-1}$ [8,34].

The presence of isolated nitrogen (C center) within the diamond is indicated by the main peaks at 1344 and $1130 \text{ cm}^{-1}$ in Fig. 6b), which are characteristic of type Ib diamonds. The shoulder at $1095 \text{ cm}^{-1}$, observed in all studied diamonds, indicates the presence of aggregated nitrogen N along with vacancies V (4 N + V), a feature typical of type IaB diamonds. This suggests that the studied diamonds are primarily of type Ib but also share some characteristics with type IaB diamonds [35,36].

A few differences among batches can be found: the peak at 1256 $\text{cm}^{-1}$, only found in sample A1, comes from the concentration of $NaN_3$. As the concentration of $NaN_3$ increases, there is a progressive increase in the peak at $1260 \text{ cm}^{-1}$, together with a decrease in the height of the
<figure><img src="image_4.png" /><figcaption>Fig. 4. XRD patterns of three different diamond batches (A1, A3 and B1) of HPHT diamond.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 5. Representative Raman spectra of four batches of HPHT yellow diamonds: A1(black), A2(red), A3(blue) and B1(green).</figcaption></figure>

<table><caption>**Table 1**
Full width at half maximum (FWHM) of the diamond band at 1331.6 cm⁻¹ for the four diamond batches.</caption><thead><tr><th>Samples</th><th>A1</th><th>A2</th><th>A3</th><th>B1</th></tr></thead><tbody><tr><td>FWHM in cm<sup>-1</sup> (band at 1331.6 cm<sup>-1</sup>)</td><td>2.86<br>(2)</td><td>2.95<br>(2)</td><td>3.05<br>(3)</td><td>3.00<br>(2)</td></tr></tbody></table>

shoulder at 1096 cm⁻¹ [10]. Therefore, the presence of NaN₃ is detected, but only in sample A1. NaN₃ is used as a dopant in HPHT [37] diamonds, which makes this observation plausible. Bands associated with C-center, at positions 1344 and 1130 cm⁻¹ and A-center at position 1256 cm⁻¹, were more pronounced in the spectra of A1 and B1 compared to A2 and A3. This suggests a slightly higher concentration of nitrogen defects in samples A1 and B1 [33]. Beyond 1130 cm⁻¹ no additional peaks related to nitrogen modes were observed. Instead, these peaks point to elements other than carbon and nitrogen bonds in the diamonds. For instance: peaks at 964 cm⁻¹ suggest the presence of C—H bonds; a peak at 881

cm⁻¹ indicates the presence of C-O-C bonds and between 600 and 700 cm⁻¹, the spectra may reveal presence of both C—H and N—H bonds [38].

### 3.6. UV spectrometry results

In order to evaluate the absorbance of light, UV spectrometry was undertaken. The results, shown in Fig. 7, reveal significant absorption in the purple and blue regions, moderate absorption in the infrared spectrum, and evident transmission and diffusion in the green and dark red ranges, with peaks in the yellow (600–620 nm), orange, and near-red (650–680 nm) wavelengths. These data confirm the presence of N and the inclusion of elements such as Fe, Ni and Cu within the diamonds, as discussed in §3.2 and §3.4.

### 3.7. Polariscop results

Through the interplay of a polarizer and an analyzer, a polariscop
<figure><img src="image_6.png" /><figcaption>Fig. 6. Overlay FTIR spectra of HPHT yellow diamonds from four different batches (A1, A2, A3, and B1) showing IR absorbance in two spectral ranges: (a) 1400–3000 cm⁻¹ and (b) 400–1400 cm⁻¹.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 7. Absorption spectra of HPHT yellow diamonds, showing absorbance in the 400–550 nm wavelength range and transmission in the 600–700 nm range.</figcaption></figure>

allows us to observe the presence of residual stress in the diamonds. No changes in brightness or strain interference patterns were observed when diamonds were rotating, meaning that all diamonds are perfectly isotropic. This confirms also that diamonds were not subjected to stress during their manufacturing processes.

## 3.8. Mechanical test results

Based on the microscopic analysis discussed in §3.1, each batch contains a specific proportion of well-shaped cuboctahedral diamonds and a proportion of irregular, damaged, or interpenetrated crystals. This distribution influences the number of cycles (NC) in the friability (or

impact) test, as described in §2.9, and consequently affects the hardness of the sample. The results of five impact tests conducted for each batch

<caption>**Table 2**</caption>

Results of friability tests on four batches of diamonds. Standard deviation can be used as an indicator of the homogeneity of the samples.

<table><thead><tr><td>Samples</td><td>Number of cycles (NC)</td><td>Standard deviation</td><td>Standard deviation (%)</td></tr></thead><tbody><tr><td>A1</td><td>3223</td><td>325</td><td>10</td></tr><tr><td>A2</td><td>3068</td><td>319</td><td>10</td></tr><tr><td>A3</td><td>2223</td><td>202</td><td>9</td></tr><tr><td>B1</td><td>2733</td><td>83</td><td>3</td></tr></tbody></table>
are presented in Table 2.

Among the samples from supplier A, batch A1 exhibits the highest NC value, indicating the greatest impact resistance, while batch A3 shows the lowest NC value. The B1 sample displays an NC value between those of A2 and A3 and has the lowest standard deviation, suggesting high consistency in mechanical behavior. Therefore, A1 is the most impact-resistant sample, whereas B1 is the most homogeneous in terms of mechanical properties and exhibits low to moderate impact resistance.

Batches A1, A2, and A3 contain more elements than batch B1 (see XRF results discussed in §3.2). This may suggest that the trace elements or inclusions in the material could create stress points (not detectable by polariscope analyses) that disrupt the crystal lattice or grain structure, potentially weakening the material after repeated tests. Additionally, the presence of elements such as Fe, W and Ni in higher concentrations in batches A1, A2, and A3 compared to B1 may increase the number of nitrogen and nitrogen-related defects. These defects, discussed in §3.4 and in §3.5, contribute to the increased standard deviation of batches A, and, consequently, reduced reliability of these samples in mechanical tests.

### 3.9. Thermal treatment results

To evaluate resistance to thermal treatment (TT) three samples, A1, A3 and B1 were heated at 680 °C for 24 h. The chosen heat treatment (680 °C for 24 h) is reported to force the graphitization [11,12].

Based on the Raman spectroscopy results discussed in §3.4, A1 and A3 samples were chosen since they exhibited the best and worst crystallinity, using as parameter the FMWH of the Raman band.

By weighing between 400 and 900 mg of diamonds per batch before and after annealing, we determined the following mass losses: 3.09(3) % for sample A1, 4.31(3) % for A3, and 4.53(8) % for sample B1. The lower mass loss observed in A1 indicates superior thermal stability at 680 °C compared to the other samples.

To investigate the origin of this mass loss and to observe potential morphological changes, we analyzed an average of 10 diamonds per batch using scanning SEM. Although these images do not allow for a rigorous statistical comparison among A1, A3, and B1, they do provide valuable insight into surface defects and the possible mechanisms behind the mass loss.

A representative SEM image of a heat-treated sample is shown in

Fig. 8. It reveals significant surface degradation, most notably the presence of large triangular features known as trigons [39]. These structures typically form on the octahedral {111} faces aligned along the crystal's [111] directions (three-fold symmetry axes of A3) and are indicative of partial chemical corrosion or dissolution [40]. Their formation is closely associated with pre-existing defects, which serve as preferential sites for etching [41].

Several trigons exceeding 30 µm in size were observed, likely due to the thermal enlargement of existing surface imperfections. This observation is consistent with a dissolution-driven mechanism, where trigon steps propagate outward over time, especially at elevated temperatures [42].

In addition to trigons, parallel, layered surface features were also observed. These are most likely related to the {001} planes of the diamond crystal, aligned with the [001] directions (four-fold symmetry axes of A4). The formation of both trigons and planar features may have been triggered by exposure to high temperatures (680 °C), possibly due to reactions with atmospheric oxygen leading to the release of CO₂ [43].

Furthermore, Raman experiments were performed to determine whether graphitization had occurred at the surface. The Raman band characteristic of sp² hybridization was not detected. The analysis of the FWHM of the diamond band for all samples showed an increase (see Table 3), providing further evidence of loss of crystallinity and thermal degradation. Sample A1 retained slightly better crystallinity.

Sample A3 and sample B1, which have similar quality (Table 2) but were synthesized differently, were subjected to additional mechanical testing following heat treatment at 680 °C.

After heat treatment, the A3 sample exhibited no significant change in NC value (Table 4). In contrast, the NC value of the thermally treated B1 sample (TT-B1) was considerably lower than that of the original B1. While B1 demonstrates more reproducible mechanical behavior under ambient conditions, it appears to be more susceptible to degradation

<caption>Table 3</caption>

Full width at half maximum (FWHM) of the diamond band at 1331.6 cm⁻¹ for A1, A3 and B1 before and after heat treatment at 680 °C.

<table><thead><tr><th>Samples</th><th>A1</th><th>A3</th><th>B1</th></tr></thead><tbody><tr><td>FWHM in cm<sup>-1</sup> (band at 1331.6 cm<sup>-1</sup>) before HT</td><td>2.86(2)</td><td>3.05(3)</td><td>3.00(2)</td></tr><tr><td>FWHM in cm<sup>-1</sup> (band at 1331.6 cm<sup>-1</sup>) after HT at 680 °C</td><td>3.51(3)</td><td>3.59(4)</td><td>3.66(4)</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>Fig. 8. Representative SEM image of a diamond from batch B1 after heat treatment at 680 °C, showing significant surface degradation. Notable features include the presence of trigons and a layered structure. Crystallographic planes and major symmetry axes are indicated on the right.</figcaption></figure>
<table><caption>**Table 4**
Results of impact test on two batches of diamonds after thermal treatment (TT) at 680 °C.</caption><thead><tr><th>Samples</th><th>Number of cycles (NC)</th></tr></thead><tbody><tr><td>A3 – after treatment at 680 °C (TT-A3)</td><td>2312</td></tr><tr><td>B1 – after treatment at 680 °C (TT-B1)</td><td>2427</td></tr></tbody></table>

upon heating. This susceptibility may explain the observed loss in mass and mechanical performance following thermal treatment. The difference in thermal stability between the samples could be attributed to variations in trace element content, as they originate from different suppliers and were synthesized under differing conditions. The higher content of carbide-forming elements (Nb, W, Mo, Cr, Y, and Zr) in samples A1 and A3 may have played a protective role by suppressing excessive graphitization [44] and limiting the degradation of the diamond powders, in contrast to sample B1.

## 4. Conclusions

The lack of detailed sample characterization from suppliers limits the understanding of variation in mechanical performance and thermal durability of commercial HPHT diamonds, as well as the price differences. In order to address this gap, four commercial HPHT diamond types were analyzed and characterized using accessible, cost-effective laboratory techniques, that do not require altering the samples.

For diamond batches with similar average crystal size, the primary factors affecting quality include: i) morphology (specifically, the proportion of regular cube-octahedral diamonds in the batch), ii) the type and quantity of inclusions from the growth process, and iii) the density of lattice defects within the material.

Table 5 summarizes the main results from various analyses. For Supplier A both quality and batch pricing primarily depend on the amount of high-quality cube-octahedral diamonds. In contrast, batch B, sourced from a different supplier, highlights the significant role of inclusions. This suggests that Supplier B's manufacturing process may be more controlled than that of Supplier A, leading to differences in trace element composition. These differences could explain the greater repeatability and stability in the mechanical behavior of Supplier B's batch. Conversely, the higher concentration of trace elements in Supplier A's samples, which inhibit graphitization, may provide an advantage in preserving diamond integrity under high-temperature conditions.

In conclusion, the properties of the four tested commercial samples, align well with the intended commercial applications stated in §2.1. The most expensive sample, batch A1, which demonstrates higher mechanical resistance and superior morphological quality is suitable for precision cutting and drilling. In contrast, batches A3 and B1, which are more economical, are intended for broader, general-purpose applications. The differences between these two batches, specifically, A3's better temperature resistance and B1's more consistent performance after repeated mechanical testing, stem from variations in the growth process and residual trace elements.

### CRediT authorship contribution statement

**Gianfranco Gobbo:** Writing – original draft, Visualization, Investigation, Formal analysis. **Quentin Lachuer:** Writing – original draft, Methodology, Investigation, Formal analysis. **Sophie Depriester:** Methodology, Investigation, Formal analysis. **Tommaso Maurizi-Enrici:** Writing – review & editing, Writing – original draft, Supervision, Methodology, Conceptualization. **Gérard Panczer:** Writing – review & editing, Methodology, Investigation, Formal analysis. **Yann Le Godec:** Writing – review & editing, Methodology, Conceptualization. **Irene Calliari:** Writing – review & editing, Validation, Supervision. **Sylvie Le Floch:** Writing – review & editing, Supervision, Methodology,

<table><caption>**Table 5**
Summary of the characteristics of four batches of microdiamonds.</caption><thead><tr><th>Samples</th><th>Ratio of cube-octahedral</th><th>Ratio of fused diamond</th><th>Inclusions</th><th>Quantity of different trace elements detected</th><th>Elements present (&gt;100 ppm)</th></tr></thead><tbody><tr><td>A1</td><td>70 %–75 %</td><td>4.7 %</td><td>Fe<sub>3</sub>C, CrC</td><td>10</td><td>Fe, Al</td></tr><tr><td>A2</td><td>55–60 % 1</td><td>7.8 %</td><td>Not Analyzed</td><td>8</td><td>Fe, W, Cr, Ni</td></tr><tr><td>A3</td><td>45–50 %</td><td>12.7 %</td><td>Fe<sub>3</sub>C, CrC</td><td>10</td><td>Fe, W, Ni</td></tr><tr><td>B1</td><td>45–50 %</td><td>11 %</td><td>Fe, Y, Fe<sub>3</sub>C, CrC</td><td>4</td><td>Fe</td></tr></tbody></table>

**Conceptualization. Vittoria Pischedda:** Writing – review & editing, Writing – original draft, Supervision, Methodology, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors acknowledge Ruben Vera from the Centre de Diffractométrie Henri Longchambon (CDHL), University of Lyon 1, for his assistance with XRD characterizations, and Nicholas Blanchard (ILM, UCBL) for his support with SEM imaging. This work was supported by the Arqus European University Alliance, the France 2030 Investment Plan and the French National Research Agency.

### Data availability

Data will be made available on request.

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