Using Spectroscopy for Advanced Gemstone Analysis

Gemologists often have two standard questions, “What are the characteristics of the stone I’m asked to measure?”, and “Are these characteristics natural?” This is because the industry has developed a number of techniques to “enhance” stones to hide imperfections and optimize color.

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Techniques used include irradiating, heat treating, laser drilling, resin filling, and even simple surface medications such as petroleum product treatments.

A typical basis system consists of an AvaSpec-ULS2048-USB2 spectrometer, white reference tile for gemology, a WS-2-GEM, and either an AvaLight-HAL halogen light source and FCR-7UV200-2-BX fiber-optic reflection probe or an AvaSphere-50-LS-HAL, reflection integrating sphere equipped with halogen illumination.

Although there are a number of potential applications, the determination of diamond types Ia or Ib status or the measurement of the main chromium peaks (692.8 nm and 694.2 nm) in rubies is often performed with this system.

Type Ia or colorless diamonds exhibit strong nitrogen absorption peaks at 415 nm and 478 nm, while type Ib or yellow diamonds do not show these peaks as they have a wider distribution of nitrogen atoms. Other useful peaks are at 592 nm and 741 nm, peaks indicating the occurrence of artificial coloring.

Advanced Gemology Measurements

The application of photoluminescence systems to determine gemstone characteristics has been reported by various entities. Most frequently, this method utilizes a sample chamber, laser excitation source, and detection system. The laser wavelength is based on the desired features.

Most of the time, the chamber will have provisions for cryogenic cooling. The detection system can range from simple cameras to photomultiplier tubes. The systems available from Avantes feature the AvaSpec-ULS2048LTEC-USB thermoelectrically cooled spectrometer and a variety of lasers.

An AvaRaman-Probe, which is a fiber-optic Raman probe, is required to filter the excitation source from the measured data. The detection of high-pressure, high-temperature treatment (HPHT) is an application demonstrating the usefulness of this method. The HPHT process is sometimes used to enhance diamond color, for example converting brown diamonds into yellow or green diamonds.

Many regulatory agencies demand that all such treatments are disclosed, but the major concern is the possibility of misrepresenting such stones as natural or untreated ones. An Avantes economical, high-performance photoluminescence system can detect the HPHT process as it leaves a trace peak at 694 nm.

Why Avantes for Gemology Applications?

• Applications experience of the configuration required to test the varied grades of diamonds
• A broad range of spectrometers are available to provide optimal and economical solutions
• Custom manufacture of probes (for example, a zero fluorescence probe)

In the gemology industry, the measurement needs are demanding for various reasons, of which low signal measurement and the need for fast measurement systems to manage the volume of gemstones measured are the most important.

Avantes offers both basic measurement tools and sophisticated systems to address a variety of gemological requirements. Relatively simple systems that use either reflection probes or integrating spheres up to photoluminescence systems that require TE-cooled spectrometers and lasers can be used to perform gemology measurements.

Basic Gemology Setup

Advanced Gemology Setup

This information has been sourced, reviewed and adapted from materials provided by Avantes BV.

For more information on this source, please visit Avantes BV.