Presidium Gem Tester II (PGT II): Identify Diamonds, Moissanites, and Colored Gemstones with Ease

Imagine you’ve inherited a beautiful ring from a beloved aunt. It shimmers with a captivating blue stone. Is it a priceless sapphire, a valuable tanzanite, or perhaps a cleverly crafted piece of glass? The world of gemstones is filled with beauty, but also with potential for confusion and even deception. Determining the true identity of a gemstone can be a significant challenge, even for experienced jewelers.

Heat’s Hidden Signature: Understanding Thermal Conductivity

One of the key properties that gemologists use to identify gemstones is thermal conductivity. This might sound like a complex scientific term, but the basic idea is quite simple. Thermal conductivity refers to a material’s ability to conduct heat. Think about holding a metal spoon in a cup of hot tea. The metal quickly heats up, right? That’s because metal has high thermal conductivity. Now imagine holding a wooden spoon in the same cup. The wood remains relatively cool. Wood has low thermal conductivity.

Different materials conduct heat at different rates because of their atomic structure. Materials with tightly packed atoms and strong bonds, like metals, tend to be good conductors. Materials with loosely packed atoms and weaker bonds, like wood or plastic, are poor conductors. Gemstones, with their diverse crystalline structures and compositions, fall along a wide spectrum of thermal conductivity.

Diamond, for example, is an exceptional conductor of heat – far better than almost any other material. This is due to its strong covalent bonds and highly ordered crystal structure. This exceptional property is one reason why diamonds feel cold to the touch: they rapidly draw heat away from your skin. Moissanite, a lab-created gemstone that closely resembles diamond, also has very high thermal conductivity, although slightly lower than that of natural diamond. Other gemstones, like quartz, topaz, and garnet, have significantly lower thermal conductivities. Glass, a common diamond simulant, has a very low thermal conductivity. This difference in how quickly they transfer heat forms the basis of thermal conductivity testing. To be more specific, diamond’s thermal conductivity is in the range of 2000-2500 W/m·K (Watts per meter-Kelvin), while moissanite’s is around 800-1000 W/m·K. Glass, on the other hand, typically has a thermal conductivity of around 1 W/m·K.

Meet the Presidium Gem Tester II: A Clever Tool

The Presidium Gem Tester II (PGT II) is a handheld device designed to leverage the principle of thermal conductivity for quick and convenient gemstone identification. It’s a popular tool among jewelers, gem collectors, and hobbyists, providing a relatively easy way to distinguish between diamonds, moissanites, and many common colored gemstones. It’s not a magic bullet that can identify every gemstone with certainty, but it’s a valuable tool in the gemologist’s arsenal.

Unlocking the Secret: How the PGT II Works

The heart of the PGT II is its retractable thermoelectric probe. This probe contains a tiny, but ingenious, device called a thermocouple.

The Curious Case of the Thermocouple

A thermocouple is essentially a junction of two different types of metal alloys. When this junction experiences a change in temperature, it generates a small electrical voltage. This phenomenon is known as the Seebeck effect.

The Seebeck Effect: Hot and Cold, Making Electricity

Imagine two wires, each made of a different metal, joined together at one end. If you heat this junction, a small voltage will be created between the two unjoined ends of the wires. The size of this voltage is directly proportional to the temperature difference between the hot junction and the cold ends of the wires. This is the Seebeck effect in action.

The PGT II’s probe tip uses this principle. When the probe touches a gemstone, a tiny heater within the probe warms the tip slightly. The gemstone then draws heat away from the probe at a rate determined by its thermal conductivity. The thermocouple measures the resulting temperature change at the probe tip, and this temperature change is translated into a reading on the PGT II’s analog dial.

Calibration: Setting the Stage for Accuracy

Before using the PGT II, it’s essential to calibrate it. This ensures that the device is giving accurate readings, taking into account the ambient temperature of your environment. The PGT II features Assisted Thermal Calibration (ATC) to simplify this process. The calibration involves touching the probe to built-in calibration discs, allowing the device to establish a baseline for different thermal conductivity levels. It’s a bit like tuning a musical instrument before playing it.

Putting it to the Test: A Step-by-Step Guide

Using the Presidium Gem Tester II is relatively straightforward:

1. Power On: Turn on the device and allow it to warm up. The warm-up period ensures the internal components reach a stable operating temperature.
2. Calibrate: Follow the manufacturer’s instructions for calibration. This typically involves touching the probe to the calibration discs in a specific sequence, as indicated on the device. The calibration process establishes a baseline for comparison.
3. Clean the Gemstone: Ensure the gemstone you’re testing is clean and free of any dirt, oil, or debris. Contaminants can interfere with the heat transfer and affect the reading. Use a soft, lint-free cloth to gently clean the stone.
4. Test: Gently but firmly press the retractable probe tip against a flat, polished facet of the gemstone. Maintain consistent pressure. Avoid pressing too hard, as this could damage the probe or the gemstone.
5. Read the Result: Observe the needle on the analog dial. The dial is marked with zones indicating different gemstone types, including diamond, moissanite, and various colored gemstones.
6. Important Note: For small stones (below 0.02ct), the results may be less reliable. It is recommended to use additional testing methods in such cases.

Beyond Diamonds: Identifying Other Gemstones

While the PGT II is excellent for distinguishing between diamonds and moissanites, it can also help identify other gemstones. The analog dial includes markings for common gemstones like sapphire, ruby, emerald, jade, and quartz. However, it’s important to note that some gemstones have overlapping thermal conductivity ranges. For example, a reading in a particular zone might indicate either topaz or spinel. In such cases, additional testing methods would be needed to confirm the gemstone’s identity.

Strengths and Limitations: A Balanced Perspective

The Presidium Gem Tester II offers several significant advantages:”

• Speed: It provides results in just a few seconds, making it a quick and efficient tool for initial screening.
• Ease of Use: The analog dial is simple to read, and the testing process is straightforward, even for beginners.
• Portability: Its compact, battery-powered design makes it easy to carry and use in various settings, from jewelry stores to gem shows.
• Diamond/Moissanite Differentiation: It excels at distinguishing between diamond and moissanite, two stones that can be visually very similar.
• Assisted Thermal Calibration: The ATC feature helps ensure accurate readings by compensating for variations in ambient temperature.
• Durability: The sturdy construction of the PGT II adds to the longevity.
• Replaceable Probe: If the probe tip becomes damaged or worn, it can be easily replaced without needing to purchase a whole new unit. This is a cost-effective feature for long-term use. The replacement probe (available separately) typically costs around $75, which is significantly less than the cost of a new tester.

However, like any testing instrument, the PGT II has limitations:

• Natural vs. Synthetic Distinction: It cannot differentiate between natural and synthetic gemstones. Since the chemical composition and crystal structure (and therefore thermal conductivity) are generally identical, the PGT II will give the same reading for a natural sapphire and a lab-created sapphire.
• Overlapping Ranges: Some gemstones have similar thermal conductivities, leading to potential ambiguity in the readings. For example, certain types of garnet and spinel may fall within the same range on the dial.
• Small Stone Limitations: The accuracy of the PGT II can be compromised when testing very small gemstones (generally less than 0.02 carats). This is because the small surface area makes it difficult to achieve proper thermal contact and obtain a stable reading.
• Surface Condition Sensitivity: The gemstone’s surface must be clean and polished for accurate results. Dirt, oil, or a rough surface can interfere with heat transfer.
• Not a definitive test: It’s an indicative test.

The Gemologist’s Toolkit: Other Testing Methods

The PGT II is a valuable tool, but it’s just one piece of the puzzle in gemstone identification. Professional gemologists rely on a combination of tests to make accurate determinations. Here are a few other essential methods:

• Refractometer: This instrument measures the refractive index (RI) of a gemstone, which is the degree to which light bends as it passes through the stone. Different gemstones have characteristic refractive indices, making this a powerful identification tool.
• Specific Gravity (SG) Test: This test determines the density of a gemstone by comparing its weight in air to its weight in water. Different gemstones have different densities.
• Polariscope: This instrument helps determine if a gemstone is singly refractive (like diamond or garnet) or doubly refractive (like sapphire or ruby). It uses polarized light to reveal the optical properties of the gemstone.
• Microscopic Examination: A microscope allows gemologists to examine a gemstone’s internal and external features, such as inclusions (internal flaws), growth patterns, and surface characteristics. These features can provide valuable clues about the gemstone’s identity and origin.
• Spectroscope: A spectroscope analyzes how a gemstone absorbs light. Different gemstones absorb different wavelengths of light, creating unique spectral patterns that can be used for identification.

A Lasting Shine: Conclusion

The Presidium Gem Tester II (PGT II) is a valuable asset for anyone who works with or enjoys gemstones. By understanding the principle of thermal conductivity and how the PGT II utilizes it, you can quickly and confidently distinguish between diamonds, moissanites, and a range of other colored gemstones. While it’s not a replacement for a full gemological laboratory analysis, it’s an excellent first step in the identification process, providing valuable information and helping to separate the genuine articles from imitations. It empowers jewelers, collectors, and enthusiasts to make more informed decisions and appreciate the fascinating world of gemstones with greater confidence. Remember to always use the PGT II in conjunction with other gemological tools and knowledge for the most accurate and reliable results. The world of gems is full of surprises, and the PGT II is a helpful guide along the way.