Using Field Spectroradiometer for Synthetic Emerald Identification

author: Callum

2024-11-25

The primary evidence for identifying synthetic emeralds relies on inclusion characteristics and infrared absorption spectra, UV-Vis absorption spectral analysis also provides supplementary evidence with certain diagnostic significance.

Figure 1 Natural Emerald vs. Synthetic Emerald

This study utilizes a field spectroradiometer to collect UV-Vis (including partial near-infrared) absorption spectra of synthetic emerald samples. By comparative analysis, the study summarizes the distinctive identification features of several common types of synthetic emeralds available in the market as observed through the field spectroradiometer.

Principles

Field spectroradiometer is an essential scientific instrument used for measuring and collecting the spectral data of terrestrial objects. It can measure physical quantities such as luminance, illuminance, reflectance, and spectral distribution. Integrating measurement, data acquisition, storage, and computation, it is a crucial tool for collecting spectral data and extracting spectral feature information.

In terms of technical features and applications, the field spectroradiometer is characterized by high sensitivity and high resolution, with a built-in fixed holographic grating and full-line array optical detector. This configuration avoids potential calibration failures caused by internal fiber splitting, narrow slits, or moving gratings/prisms.

Figure 2 Principle of Field Spectroradiometer

The field spectroradiometer has extensive applications, including:

Mineral Exploration: It is used for mineral identification and resource estimation as different minerals have unique spectral characteristics, allowing for the identification of types and distributions through spectral analysis.
Remote Sensing Applications: In land cover classification, satellites or UAV-mounted spectroradiometers are used to classify and monitor surfaces such as forests, farmland, and urban areas.
Disaster Monitoring: Spectral analysis enables the rapid assessment of the scope and severity of natural disasters.

With its advanced features and broad applications, the field spectroradiometer plays a critical role in various domains, providing reliable and efficient spectral data collection and analysis.

Workflow

The test samples for this study include three natural emeralds: NE01, NE02, and NE03; five hydrothermal synthetic emeralds: SE01, SE02, SE03, SE04, SE05; and one flux-grown synthetic emerald: SE06.

1.1. Pre-test Preparation

First, configure the necessary equipment based on the testing requirements and bring the following materials:

No.	Name	Model	Quantity	Unit
1	Field Spectroradiometer	ATP9100UV	1	Unit
2	Field of View Lens	25°	1	Piece
3	Standard Reflectance White Board	95% Reflectance	1	Piece
4	Halogen Lamp	-	4	Pieces

Figure 3 Field Spectroradiometer

1.2. Data Collection and Analysis

1.2.1. UV-Visible-NIR Absorption Spectral Features of Natural Emeralds

The color-causing elements in emeralds are the main factors influencing their UV-visible absorption spectra. Among them, Cr³⁺ absorption peaks are found near 273 nm, which are caused by the 4A₂ → 4T₁(4P) transition; near 427 nm, absorption is due to the 4A → 4T₁(4F) transition; and near 612 nm, absorption is associated with the 4A₂ → 4T₂(4F) transition.

In the three natural emerald samples (NE01, NE02, NE03) tested:

Figure 4 Natural Emeralds NE01, NE02, NE03 UV-Vis-NIR Absorption Spectra

(1) The wide absorption edge in the UV region for the three samples was located at 301 nm, 315 nm, and 331 nm, respectively. This is caused by the O₂⁻ → Fe³⁺ charge transfer. As the Fe³⁺ content increased in NE01, NE02, and NE03, a red shift occurred in the absorption edge of the oxygen-iron charge transfer band. Among them, NE03 had the highest Fe³⁺ content, showing a 367 nm absorption peak, which is attributed to the Fe³⁺ 6A₁ → 4E(D) transition.

(2) The three samples exhibited progressively increasing broad absorption peaks in the range of 823-830 nm. The attribution of this peak is debated. One view suggests it is caused by the Fe²⁺ 5T₂g → 5Eg transition; another suggests it is due to the charge transfer between Fe²⁺ and Fe³⁺; while another points to the hydration ion absorption of Fe³⁺. Research indicates that this absorption peak at this position could result from a combination of Fe³⁺ d-d electronic transitions and Fe²⁺-Fe³⁺ charge transfer. All views generally agree that this peak is iron-related, and the absorption characteristics at this position in natural emeralds have certain identification significance.

（3）All three samples showed absorption near 940 nm, which is attributed to the secondary harmonic vibration of water.

1.2.2. UV-Visible-NIR Absorption Spectral Features of Hydrothermal Synthetic Emerald

1.2.2.1. Features of SE01, SE02, SE03

Figure 5 Hydrothermal Synthetic Emeralds SE01, SE02, SE03 UV-Vis-NIR Absorption Spectra

（1） In the UV region, the absorption cutoff edge for all three samples is less than 290 nm, with no observable oxygen-iron charge transfer band. This indicates that these three samples do not contain iron ions. Based on the rule that natural chromium-containing minerals must also contain iron, this suggests that these three emeralds are laboratory-synthesized. This feature can be used as a distinguishing characteristic for synthetic emeralds that do not contain iron.

（2）SE01 and SE02 show no characteristic absorption near 820 nm, while SE03 displays a broad absorption between 580 and 780 nm, which is attributed to the absorption caused by V³⁺ and Cu²⁺.

（3）An absorption peak associated with water is observed near 940 nm for all three samples.

1.2.2.2. Features of SE04, SE05

Figure 6 Hydrothermal Synthetic Emeralds SE04 SE05 UV-Vis-NIR Absorption Spectra

(1) The absorption cutoff edges in the UV region for SE04 and SE05 are 339 nm and 335 nm, respectively. There is an absorption peak between 300 nm and 345 nm due to the oxygen-iron charge transfer band, and an absorption peak around 369 nm caused by the Fe²⁺ 6A1g→4E(D) transition. This indicates that both samples contain a certain amount of iron ions.

(2) SE04 exhibits a broad absorption peak centered around 753 nm, which is caused by the Fe³⁺ 6A1g→4T2g forbidden transition. This feature can be used as a distinguishing characteristic for iron-rich synthetic emeralds. SE05 shows a broad absorption between 580 nm and 780 nm, attributed to the absorption caused by V³⁺ and Cu²⁺.

(3) An absorption peak associated with water is observed near 940 nm for both samples.

1.2.2.3. UV-Visible-NIR Absorption Spectral Features of Flux Method Synthetic Emerald

Figure 7 Flux-Growth Synthetic Emerald SE06 UV-Vis-NIR Absorption Spectra

For the single flux method synthetic emerald tested, there is no absorption peak for water. However, recent studies on flux method synthetic emeralds have shown trace amounts of water in both their infrared and UV-visible absorption spectra. In contrast, most natural emeralds also show very weak water absorption peaks in the UV-visible spectra. Therefore, it is difficult to make an accurate judgment on flux method synthetic emeralds based solely on UV-visible spectrometry. A more accurate identification should be based on the inclusion characteristics and infrared spectral features.

Conclusion

(1) The presence of the oxygen-iron charge transfer absorption band between 300 nm and 345 nm, caused by the O²⁻→Fe³⁺ charge transfer, can be used to distinguish hydrothermal synthetic emeralds without iron, such as Cr-type (SE01), Cr+V-type (SE02), and V+Cu-type (SE03).

(2) The appearance of a broad absorption peak near 760 nm, resulting from the 6A1g→4T2g forbidden transition of Fe²⁺, can be used to identify the newly marketed Russian-type hydrothermal synthetic emeralds. These new hydrothermal synthetic emeralds have inclusion and infrared spectral features very similar to Colombian emeralds. This UV-visible absorption characteristic serves as an important basis for distinguishing this type of emerald.

(3) Recent infrared and UV-visible absorption spectra of flux method synthetic emeralds show trace amounts of water. Most natural emeralds also exhibit very weak water absorption peaks in the UV-visible spectrum when measured with a field spectroradiometer. Therefore, an accurate judgment of flux method synthetic emeralds cannot be made based solely on UV-visible spectrometry. Identification should be made by considering inclusion features along with infrared spectral characteristics.