Diamond vs. Cubic Zirconia (CZ): How to Tell the Difference?

If someone offers you a “diamond” at a surprisingly good price, it’s smart to pause and take a closer look. Many stones that appear diamond-like are actually cubic zirconia (CZ)—a convincing imitation that costs a fraction of the price.

So how can you tell the difference between a real diamond and CZ? Here’s what you need to know.

What Is Cubic Zirconia (CZ)?

Cubic zirconia is a man-made gemstone with the chemical name zirconium dioxide. While it can occur naturally in very small quantities, nearly all CZ stones on the market are laboratory-created.

CZ is widely used as a diamond substitute because it is:

• Colorless or near-colorless

• Highly sparkly

• Inexpensive to produce

However, despite its appearance, CZ is not a diamond and does not share the same physical or optical properties.

Hardness Comparison

• Diamond: 10 on the Mohs hardness scale (hardest natural substance)

• Cubic Zirconia: Approximately 8–8.5 on the Mohs scale

This means CZ scratches and wears much more easily over time.

How to Tell the Difference Between a Diamond and CZ

While professional testing is the most reliable method, there are several signs you can look for on your own.

1. The Fog Test

Breathe gently on the stone.

• Real diamond: Fog disperses almost instantly due to high heat conductivity

• CZ: Fog lingers for a few seconds

2. Examine Under Magnification

Use a jeweler’s loupe or magnifying glass.

• Diamond: Typically contains natural inclusions or imperfections

• CZ: Often appears perfectly clean and flawless

Note: Exceptionally high-grade diamonds can also appear very clean, so this test alone is not conclusive.

3. Weight Test

CZ is significantly denser than diamond.

• A CZ stone weighs about 1.5–2× more than a diamond of the same size

If the stone feels unusually heavy, it may be CZ.

4. Sparkle and Color

• Diamond: Reflects mostly white light with subtle flashes

• CZ: Displays more intense rainbow colors and excessive fire

Too much color can actually be a red flag.

5. Facet Sharpness

• Diamond: Extremely sharp, crisp facet edges

• CZ: Slightly rounded edges due to lower hardness

6. Setting Quality

Consider the jewelry itself.

• High-quality diamonds are rarely set in cheap or lightweight mountings

• Low-quality settings often indicate imitation stones

Should You Try a Scratch Test?

While diamonds are extremely scratch-resistant, scratch testing is not recommended. It can damage the stone or the jewelry and isn’t necessary for identification.

The Most Reliable Test: A Professional Jeweler

The safest and most accurate way to identify a diamond is to have it tested by a reputable jeweler or gemologist. They use specialized tools such as:

• Diamond testers (thermal and electrical conductivity)

• Microscopes

• Laboratory certification verification

If a seller refuses professional testing, that’s usually your answer.

Final Thoughts

Cubic zirconia can be beautiful and affordable—but it should never be sold as a diamond. Understanding the key differences helps you avoid overpaying and shop with confidence.

When in doubt, trust certification, professional testing, and reputable sellers. A real diamond is a long-term investment, and knowing what you’re buying makes all the difference.