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In movies there seems to be an easy method to find out if a diamond is real or fake:

If a diamond is able to scratch/cut glass, then it's the real deal

But does that simple test work in real life?
Are fake diamonds generally not able to scratch glass?

My Questions:
What materials are commonly used for faux diamonds ?
And are they hard enough to scratch glass?

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Claims in movies aside, I thought that it wasn't just 'only a diamond can scratch glass' but 'only a diamond can scratch glass without damage to itself'. I personally have doubt that even this claim is true, but is probably closer to the truth. – iandotkelly Feb 7 '12 at 15:58
This sounds more like a physics question. Anything harder than glass could potentially scratch it. Glass has a Mohs hardness of 5 and diamond has a hardness of 10. So anything above 5 could potentially scratch glass. – GordonM Dec 16 '15 at 9:16
up vote 26 down vote accepted

Interesting question. A quick search reveals lots of people claiming that only diamonds cut glass, and a lot of people claiming that many other stones will.

When it comes to scratchability, Moh's Scale is a good measure.

The Mohs scale of mineral hardness is based on the ability of one natural sample of matter to scratch another.

Under this scale, Diamond (and variants) sit at the top at 10 while glass is at 5.5. [Ref].

Cubic Zirconia is a common faux diamond, which is still fairly new (probably ante-dating this test for diamonds).

economically important competitor for diamonds since commercial production began in 1976.

Cubic Zirconia sits in between at around 7.5-8.5 on the Mohs scale (Ref, Wikipedia puts it at 8, but the source refers to (generic) Zirconia, and I don't know if Cubic Zirconia has the same hardness.)

As Mohs Scale is based on the ability to scratch lower numbered minerals, it seems cubic zirconia can scratch glass.

It would be nice to get experimental confirmation of this theory, but I was unable to find any evidence (e.g. videos) of people actually trying it.

For other faux diamonds, see some descriptions here.

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Incidentally, I understand moissanite aka silicon carbide is the "faux" diamond that is closest to "real" diamonds. It is 9.5 on the Mohs scale. As well there is the Aggregated diamond nanorods aka "hyperdiamond" which are ">10" on the mohs scale, according to Wikipedia. – Brian M. Hunt Feb 7 '12 at 14:49
The last link is sponsored by a moissannite retailer, so they get a mention there. I put "Diamonds (and variants)" to cover hyperdiamonds, without getting too distracted. – Oddthinking Feb 7 '12 at 15:03 says no, and I've no reason to doubt them.

Diamonds cut glass, but this does not positively identify a diamond because other materials, such as quartz, also lie above glass on the Mohs scale and can also cut it.

Of course this doesn't cover artificial diamonds per se, which I think you're referring to. Those can indeed have different properties from "real" diamonds

The properties of synthetic diamond depend on the details of the manufacturing processes, and can be inferior or superior to those of natural diamond; the hardness, thermal conductivity and electron mobility are superior in some synthetic diamonds (either HPHT or CVD)

The method used to identify synthetics from natural is apparently quite different:

The appearance of synthetic gems on the market created major concerns in the diamond trading business, as a result of which special spectroscopic devices and techniques have been developed to distinguish synthetic and natural diamonds.

Synthetic diamond is the hardest material known,[53] where hardness is defined as resistance to scratching and is graded between 1 (softest) and 10 (hardest) using the Mohs scale of mineral hardness

references which compares hardness of fullerite (synthetic diamond) to natural diamond, concluding indeed that

Hardness and wear of ultra- and superhard fullerites and diamond were measured in the present study. The diamond surface (111) was deformed as a plastic material under the scratching with the ultrahard fullerite C60 indenter. This indicates that the hardness of ultra-hard fullerite is sufficient to create a high pressure in the contact point for the plastic flow of diamond at room temperature and it exceeds the hardness of diamond.

So no, a simple test of scratching glass won't identify a synthetic diamond from a natural one. In fact the synthetic may well scratch things the natural one can't scratch!

Of course other materials may be used to try to fool people into believing they're holding a diamond. Wikipedia mentions that

A diamond simulant is a non-diamond material that is used to simulate the appearance of a diamond, and may be referred to as diamante. Cubic zirconia is the most common.

materials that may well be less hard than diamond (and maybe giving rise to the myth, as hinted at in the same article).

Early diamond identification tests included a scratch test relying on the superior hardness of diamond. This test is destructive, as a diamond can scratch diamond, and is rarely used nowadays.

The Wikipedia article on diamond is surprisingly well referenced and worth reading.

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