Why Your Presidium Gem Tester Gives False Readings (A Mentor's Guide)

You just spent nearly $300 on a professional-grade tool, the Presidium Gem Tester II (PGT II). You unbox it, meticulously follow the instructions, and… the readings are all over the place. A known ruby tests as “glass.” A piece of glass tests as “calcite.” The needle won’t calibrate, or it gets stuck on “diamond.”

You’re frustrated. You feel cheated. You’re convinced the machine is faulty and you’re about to send it back.

Welcome to the club. As your mentor, I’m here to ask you to stop, and put away that return label.

You are not alone. This is the single most common experience for new owners of this device. The problem, as one brilliant user (Daniel Marquez) discovered after almost returning his, is not the machine. It’s user error.

You’re not using a simple “pass/fail” diamond tester. You’ve purchased a sensitive scientific instrument that demands a specific technique—a technique the manual fails to properly emphasize.

The #1 Secret to Accurate Readings: TOUCH, Don’t PUSH

This is the entire lesson. If you learn nothing else, learn this.

The PGT II has the “industry’s thinnest retractable probe tip.” Your instinct is to push this tip into the stone until it retracts, like you’re pressing a button. This is 100% wrong and is the reason for all your bad readings.

As user Daniel Marquez discovered, the correct technique is to “simply TOUCH the area… No pressure whatsoever!”

Why? Let’s dive into the simple science.

Lesson 1: You’re Measuring Heat Speed, Not “Gems”

This tool is not “reading the gem.” It is a thermoelectric probe. Its job is to measure one simple thing: thermal conductivity, or how quickly a material pulls heat away from the probe tip.

• The Science: The probe tip contains a tiny, precise heater. When you touch it to a stone, the stone begins to “suck” heat away. The machine measures the rate of this heat loss and moves the needle on the dial.
• The “Heat Vacuum” Analogy:
  • Diamond is the ultimate heat vacuum. It sucks heat away extremely fast. The needle flies to the “DIAMOND” section.
  • Glass is a “heat insulator.” It barely sucks any heat at all. The needle stays low, in the “GLASS” section.
  • Sapphire, Ruby, Topaz, etc., all fall somewhere in between, each with its own unique “heat vacuum” speed.

Lesson 2: Why “Pushing” Destroys Your Reading

Now, knowing it’s a heat-transfer test, think about what happens when you push the probe hard into the stone:

1. You change the contact area: You’re not just touching the tiny 0.6mm tip; you’re mashing the metal cone around the tip against the stone. This provides a massive, alternate path for heat to escape, completely corrupting the test.
2. You introduce a heat sink: If you’re holding a loose stone, your own fingers are a giant heat sink. Pushing the stone into the probe also pushes it into your fingers, warming the stone and interfering with the test.

The “no pressure” rule is the key to a consistent, repeatable test. You must let the tiny, calibrated tip alone do the work.

A Mentor’s Guide: How to Calibrate and Test (The Right Way)

Let’s throw out the confusing instructions and use the hard-won wisdom of experienced users.

The Golden Rule: At every step, the probe should be held at a perfect 90-degree angle to the surface and GENTLY TOUCHED to the target. No pressure.

1. Warm-up: Turn the machine on. Let it warm up until the “Ready” light comes on.
2. Calibration - Step 1 (CAL): Gently touch (don’t push!) the probe tip to the large metal “CAL” disc. Hold it there. The machine will beep, and the needle should climb and stabilize in the “CAL” zone on the dial.
3. Calibration - Step 2 (GLASS): This is the step everyone fails. Immediately (while the “Ready” light is still on) gently touch the probe tip to the small glass calibration disc. The needle must drop and stabilize in the “GLASS” zone.

4. Analyze Calibration:

   • If the needle lands on “GLASS,” your machine is calibrated. You’re ready to test.
   • If it doesn’t land on “GLASS” (as user Janine N. experienced), your first calibration was bad, likely due to too much pressure. Turn the machine off, let it reset, and start again from Step 1 with an even lighter touch.

5. Test Your Stone: Clean the stone’s surface. Hold the probe at a 90-degree angle and gently touch a flat facet. Hold it for 2-3 seconds. The needle will climb and settle on its true thermal conductivity.

Lesson 3: What This Tool IS and IS NOT

Now that you’re getting accurate readings, you must learn to interpret them. This is the final step to becoming a pro. As user Daniel Marquez said, “DO NOT THINK THIS IS AN END ALL BE ALL LITTLE TESTER… This is a step in the process… Do not expect it to make you a gemologist overnight.”

This Tool IS: * A Fantastic Glass Detector: As user Mark White noted, its primary job is to tell you if a stone is glass. Glass has very low thermal conductivity, so it will always read at the bottom of the scale. * A Great Diamond/Moissanite Separator: It easily separates diamond (highest reading) and moissanite (second highest) from everything else. * A “Clue-Generator”: As user James D explains, if a blue stone lands in the zone for “Tourmaline, Amethyst, and Aquamarine,” it has narrowed down your choices, but it has not given you a final answer. You now know you need to use other tools (like a refractometer) to tell those three apart.

This Tool IS NOT: * A “What Is This?” Machine: It cannot tell the difference between stones with similar thermal conductivity. A sapphire and a piece of high-quality jade might read in the same zone. * A Natural vs. Lab-Created Detector: As Mark White correctly states, a lab-created sapphire has the exact same chemical composition and thermal conductivity as a natural one. This tool will (and should) read them as identical. * An “Everything” Tester: It does not work on organic gems (like opal, amber, or pearl) because their structure is not crystalline.

Your Mentor’s Conclusion

You haven’t purchased a “magic wand.” You’ve purchased a powerful, sensitive, and somewhat finicky piece of scientific equipment. Its entire function is based on a delicate heat-transfer test, and you have been (unintentionally) corrupting that test by using too much force.

Your Presidium Gem Tester II is not broken. It’s just waiting for you to learn its language. Go back to your machine. Turn it on. Calibrate it again, but this time, with the gentle touch of an expert.

You’re not just a collector anymore. You’re a diagnostician. Welcome to the world of gemology.