Is this diamond fake or real?

Adapted from a forum posting originally contributed by rhino of GoodOldGold.com, and others

A simple test

Here is a simply little test anyone can perform to help determine if what they are looking at is a real diamond or a CZ.

First, take a blank piece of paper and make a dot on it with a pen:

Next, take the stone in question and turn it upside down on the paper. We will be positioning the diamond, table down and centered on the dot.

When you center the gemstone on the dot...

In a CZ you will see a circular reflection of that dot clearly through the pavilion. The stone on the left is the CZ. The stone on the right is a real diamond. A diamond will break up that reflection to the point where it is almost imperceptible. In a CZ it will be very easy to see.

There are other ways of making the distinctions between fakes and real but this is an easy simple test that anyone can perform.

What if the diamond is mounted?

Unless you have special equipment, determining whether a mounted diamond is real or not is much harder than a loose one. However, there are a few things you can do to help 'ease your mind' - though they are NOT as accurate as the paper test.

1. Scratching/scuffing. As we all know diamonds are really quite hard, while CZ really isn't. If you're looking at a ring and can get a loupe or magnifiying glass, have a good look at the facets of the stone (tilt it so that the reflection is completely on the surface). CZs of any age will almost definitely show light scratching and scuffing, whereas diamonds are very unlikely to do so.
2. Sharp edges. Since it's possible to cut a diamond much more 'finely' than a CZ, the edges between the facets will always be very sharp on a diamond. Obviously, experience is rather helpful here because otherwise you don't know what you should be comparing to, but if you have a chance to look at both together you can clearly see differences.

The photo below shows a stone that's been around a bit. In this photo you can easily make out scuffs/abrasions at the facet junctions as well as scratches on the table.

Here is a CZ alongside an AGS ideal cut to demonstrate the differences. Also, notice that CZ's are generally cut pretty sloppy when it comes to symmetry characteristics.

Other tests

Weight. If you have a precision scale, another great test for an unmounted CZ/diamond is weight. A CZ of the same size as a diamond will weigh nearly twice as the diamond. Remember, a 1.0ct (real) diamond weighs 200 milligrams.

Gizmos. The easiest way to reliably test is to use a diamond tester. The typical tester measures thermal and electrical conductivity. The more advance testers also measure reflectivity, and the top-of-the-line testers check both.

Myths: The NON-tests

The following so-called "tests" are not accurate ways to tell fake from real diamonds:

• It scratches glass! CZ scratches glass. Also, think twice whether you really want to submit your expensive, rare gemstone to that kind of treatment.
• It glows under UV light. Some diamonds fluoresce, and some do not. Likewise, fake gemstones can be doped or treated in such a way that they will glow under UV light.

The best way: Ask your friendly neighborhood jeweler

Take a trip to your friendly neighborhood jeweler (preferably a real jeweler and not a chain at the mall) with the gemstone in question. Most jewelers walk around with a diamond tester and a loupe in their pocket, and will gladly check your gemstone without charging you.