Side by side
| Property | Diamond | Cubic zirconia |
|---|---|---|
| Mohs hardness | 10 | 8 to 8.5 |
| Specific gravity | 3.52 | 5.6 to 6.0 |
| Refractive index | 2.42 | 2.15 |
| Dispersion (fire) | 0.044 | 0.060 |
| Thermal conductivity | Exceptional | Poor |
| Ages well | Indefinitely | Clouds and scuffs in a year or two |
| Rough price per carat | Thousands | A few dollars |
1. The weight is wrong
This is the fastest check if the stone is loose. CZ is around 60 percent denser than diamond, so a CZ cut to the same dimensions as a one carat diamond weighs closer to 1.6 carats. In the hand it feels heavy for its size in a way that is hard to describe but easy to notice once you know to look for it.
2. It holds fog
Diamond is the best thermal conductor of any natural material and CZ is an insulator. Breathe on both and the diamond clears in about a second while the CZ stays misted for two to four. This works on mounted stones, which makes it the most practical check for a ring you are wearing.
3. You can almost read through it
At a refractive index of 2.15, CZ bends light hard but not as hard as diamond. Place a loose stone flat side down on printed text: through a diamond you see nothing at all, through a CZ you will usually catch distorted shapes or light coming through. Any ambiguity in that result is itself informative.
4. Too much rainbow
CZ disperses light about 35 percent more than diamond does, so it throws more colour and less white brilliance. In sunlight it can look almost gaudy next to a diamond of the same cut. On its own this is a soft signal, since cut quality affects it heavily, but it is a reasonable first impression.
5. It ages badly
The most reliable tell on any piece more than a couple of years old. Everyday dust contains quartz at Mohs 7, which does not touch a diamond but slowly abrades a CZ at 8 to 8.5. Look at the facet edges: rounded, scuffed edges and a milky surface mean CZ that has been worn. A stone that looks as crisp as the day it was set is a good sign.
Not sure what you are looking at?
Photograph the stone and the app gives you a visual read on the 4Cs, an estimated value range, and what to check next. Free on iOS and Android.
Frequently asked questions
Is cubic zirconia heavier or lighter than diamond?
Does cubic zirconia get cloudy over time?
Can a jeweller tell instantly?
Is CZ worth anything?
Worth being clear: everything here, and the app itself, is an educational reference. At-home tests narrow things down, they do not certify a stone. Before you buy, sell or insure anything of real value, get it looked at by a certified gemologist or an independent appraiser.