How Can You Tell If It's a Diamond or Cubic Zirconia?

Diamonds have captivated humanity for centuries with their brilliance and rarity, but their high price tag has led to the popularity of diamond simulants like cubic zirconia. While cubic zirconia offers visual appeal at a fraction of the cost, distinguishing between these two gemstones is crucial for consumers making purchasing decisions or evaluating existing jewelry. This comprehensive guide outlines the key differences between diamonds and cubic zirconia and provides practical methods to identify each.

Physical Properties: The Fundamental Differences

Hardness and Durability

Diamonds earn their reputation as "forever" stones by ranking 10 on the Mohs scale of hardness—the hardest naturally occurring substance on Earth. This exceptional hardness allows diamonds to maintain their polish and resist scratching over generations.

Cubic zirconia, while reasonably durable at 8-8.5 on the Mohs scale, is significantly softer than diamond. Over time, a cubic zirconia will develop small scratches and abrasions on its surface, gradually diminishing its brilliance and clarity.

Weight and Density

One of the most reliable distinctions between these gemstones lies in their density. Cubic zirconia is approximately 1.7 times heavier than diamond when comparing stones of identical size. A one-carat diamond will appear noticeably larger than a one-carat cubic zirconia due to this difference in density.

Thermal Conductivity

Diamonds possess exceptional thermal conductivity, dispersing heat rapidly throughout the stone. Cubic zirconia, in contrast, retains heat much longer. This property forms the basis for thermal conductivity tests used by jewelers to identify diamonds.

Visual Characteristics: What Your Eyes Can Tell You

Light Dispersion and Brilliance

While both stones sparkle beautifully, they interact with light differently:

• Diamonds produce what gemologists call "brilliance" (white light reflection) and "fire" (colored light dispersion) in a balanced, distinctive pattern. Diamond's refractive index of 2.42 creates a unique visual effect that gemologists and experienced observers can recognize.
• Cubic zirconia typically displays more pronounced "rainbow" flashes (fire) than diamonds due to its higher dispersion rate. While attractive, this excessive fire can appear unnatural compared to diamond's more balanced optical performance.

Color Properties

Natural diamonds often contain slight color tints, commonly in yellow or brown hues. Only the highest quality (and most expensive) diamonds achieve true colorlessness.

Cubic zirconia is manufactured to be perfectly colorless, equivalent to the highest diamond color grades. This perfect colorlessness, ironically, can be a clue that you're looking at cubic zirconia, especially if the stone is large and priced affordably.

Clarity Characteristics

Nearly all natural diamonds contain tiny internal features called inclusions—nature's fingerprints that formed during the diamond's creation billions of years ago.

Cubic zirconia is manufactured in controlled laboratory conditions to be internally flawless. The absence of any inclusions in a supposedly "natural" stone should raise suspicions, particularly if the price seems too good to be true.

Professional Testing Methods

Thermal Conductivity Testers

Professional jewelers use electronic thermal conductivity testers (diamond testers) that measure how quickly a stone dissipates heat. These devices can quickly differentiate between diamond and most simulants, including cubic zirconia.

Specific Gravity Testing

Gemologists can perform specific gravity tests that measure density differences. The pronounced weight difference between diamond and cubic zirconia makes this test quite reliable.

Advanced Spectroscopy

For definitive identification, laboratory equipment like Raman spectroscopy can analyze a stone's molecular structure, providing conclusive identification.

DIY Testing Methods

The Fog Test

Breathe on the stone to create condensation. Due to diamond's superior thermal conductivity, the fog will disperse almost immediately. On cubic zirconia, the fog will linger noticeably longer.

The Newspaper Test

Place the stone face-down on newspaper or small text. Through a genuine diamond, you should not be able to read the text or see distinct letters due to its strong light refraction. Cubic zirconia's different optical properties may allow some visibility of the text beneath.

The UV Light Test

Under ultraviolet light, most diamonds will exhibit some degree of fluorescence (typically blue), while cubic zirconia generally remains inert.

The Water Test

Drop the stone in a glass of water. Due to its higher density, cubic zirconia will sink more quickly than a diamond of comparable size.

Context Considerations

Pricing

Perhaps the most obvious indicator is price. If a "diamond" is offered at a fraction of market value without a proper certification, it's almost certainly not a natural diamond.

Setting and Era

Consider the historical context of your jewelry. Cubic zirconia wasn't commercially available until the 1970s, so any piece genuinely dating before this period wouldn't contain cubic zirconia.

Making an Informed Decision

Both diamonds and cubic zirconia have their place in the jewelry market. Natural diamonds offer unmatched durability, investment value, and the mystique of being formed over billions of years deep within the Earth. Cubic zirconia provides impressive visual appeal at an accessible price point, allowing more people to enjoy beautiful jewelry.

The choice between diamond and cubic zirconia ultimately depends on your priorities—budget, durability, environmental and ethical considerations, and personal meaning all play important roles in this decision.

For absolute certainty regarding any valuable stone, consulting a certified gemologist remains the most reliable approach. Professional certification from respected laboratories like GIA (Gemological Institute of America) provides definitive identification and quality assessment, ensuring you know exactly what you're purchasing or owning.

By understanding the distinctive properties of both stones, you can make informed decisions whether you're shopping for new jewelry or evaluating pieces already in your possession.