Why Diamond Testers Often Get It Wrong: Lab-Grown Diamonds vs. Moissanite vs. Natural Diamonds

September 18, 2025

Introduction

Introduction

As lab-grown diamonds and simulants like moissanite become more popular, handheld diamond testers have become go-to tools for quick verification. But how accurate are they really? In this article, we'll explore the science behind diamond testing, reveal the limitations of popular devices, and explain why visual inspection under natural light may sometimes offer the clearest answer.
Section 1: What Are Lab-Grown Diamonds?

Section 1: What Are Lab-Grown Diamonds?

Lab-grown diamonds are genuine diamonds—they are chemically, physically, and optically identical to natural diamonds. Created using HPHT (High Pressure High Temperature) or CVD (Chemical Vapor Deposition) processes, they share the same core characteristics:

- Thermal Conductivity: ~2200 W/m·K (same as natural diamonds)

- Electrical Conductivity: None

Because of these identical properties, lab-grown diamonds cannot be reliably distinguished from mined diamonds using traditional handheld testers.

Section 2: Understanding Moissanite

Moissanite is a diamond simulant made of silicon carbide. It resembles a diamond but has distinct physical and optical properties:

- Thermal Conductivity: 1200–1500 W/m·K (lower than diamond, but still relatively high)

- Electrical Conductivity: Slightly conductive

- Optical Behavior: Reflects mostly white light with less dispersion than diamonds

Due to its unique combination of properties, moissanite often confuses common diamond testers—frequently resulting in false positives.

Section 2: Understanding Moissanite
Section 3: How the Presidium Multi Tester III Works (and Fails)

Section 3: How the Presidium Multi Tester III Works (and Fails)

The Presidium Multi Tester III is widely used by jewelers to test:

- Thermal Conductivity

- Electrical Conductivity

While it's designed to differentiate diamonds from moissanite, its accuracy is limited by several factors:

- Poor performance on mounted stones, especially small pavé diamonds

- Inconsistent readings depending on pressure, contact angle, and proximity to metal settings

- User technique greatly affects accuracy

Section 4: GIA iD100® — Advanced but Limited

The GIA iD100® uses laser spectroscopy to analyze a diamond's crystal growth structure and identify whether it's natural.

- Displays "PASS" for confirmed natural diamonds

- Displays "REFER" for lab-grown diamonds or moissanite

This limitation can be confusing to both buyers and sellers—particularly for high-quality, lab-grown stones.

Section 4: GIA iD100® — Advanced but Limited
Section 5: Why Visual Inspection Can Be Surprisingly Reliable

Section 5: Why Visual Inspection Can Be Surprisingly Reliable

Sometimes, your own eyes—under the right lighting—can outperform machines:

- Moissanite shows mainly white flashes

- Lab-grown and natural diamonds display rainbow "fire" and intense dispersion

Visual Tip:

View your stone under direct sunlight or a strong LED light. Diamonds (both natural and lab-grown) will exhibit colorful fire; moissanite will appear more white and glassy.

Section 6: What You Can Actually Trust

Method Recommended✅/❌ Description
Submit to a certified lab (GIA, IGI)
100% Reliable
The most accurate and authoritative way to confirm natural or lab-grown origin.
Visual inspection (natural/LED light)
Practical Tip
Great for spotting differences in fire and brilliance between stones.
Handheld testers(e.g., PMT III)
Not Recommended
Easily affected by metal settings, pressure, and angle—especially for pavé or mounted stones.
Method Recommended✅/❌ Description
Method Submit to a certified lab (GIA, IGI)
Recommended✅/❌ 100% Reliable
Description The most accurate and authoritative way to confirm natural or lab-grown origin.
Method Visual inspection (natural/LED light)
Recommended✅/❌ Practical Tip
Description Great for spotting differences in fire and brilliance between stones.
Method Handheld testers(e.g., PMT III)
Recommended✅/❌ Not Recommended
Description Easily affected by metal settings, pressure, and angle—especially for pavé or mounted stones.

Conclusion

Not all testers are created equal. Devices like the Presidium Multi Tester III and even the advanced GIA iD100 have real limitations—especially with mounted and lab-grown diamonds. Understanding their shortcomings empowers you to make smarter decisions.

When in doubt: trust your eyes, trust the light, and trust certified labs.

And remember—moissanite and lab-grown diamonds may appear similar at a glance, but they are fundamentally different. At DiamAura, we use only certified, high-quality lab-grown diamonds for every piece of jewelry we create.

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