How can I test if a ruby is real? I purchased a $10\ \mathrm{mm}\times12\ \mathrm{mm}$ synthetic ruby online for 2\$. It looks similar to cubic zirconia (CZ). Are there any physical properties of a ruby I can test at home to determine whether it's real or not?
This is an image of the back side of a CZ:

This is the back side of the ruby:

Photo quality isn't that great. In reality, they are much prettier.
 A: There are several ways you can test them. From easiest to most difficult:
Hardness
Warning, there is a small chance to damage your gemstone! It's not advisable but in case you want to try here's how to do it.
You can scratch with the tip at the back gently a very hard surface. For example CZ (~8 hardness on Mohs scale) and rubies (9 hardness) have no problem scratching glass (~5.5), or even steel (7.5-8). If you try scratching a ruby with the tip of CZ though nothing will happen, since the ruby is much harder. If your gems can scratch each other then their hardness is about the same.
(In case you do try this, try the back of your gems. That way if you manage to scratch it, it will be less visible. Also keep in mind that even though paper can't scratch glass, throwing a paper pack milk on a window can break it. In other words, hard does not mean indestructible.)

Violet laser pointer
The one I used is $405nm\ \pm10, \ <5mW$ (power matters only for eye safety). When using the laser on CZ nothing happens (no laser/with laser images respectively):
 $\longrightarrow$ 
On the other hand when hitting the ruby with a violet laser it shines brightly in red:
 $\longrightarrow$ 
(Again, photo quality isn't that great. They actually look slightly different. Also, some green lasers I tried had no effect; their wavelength was probably too long. Violet laser works, perhaps blue might work as well.)

Brewster angle
(In order to conduct this test your gemstone must have a sufficiently large flat surface. It's also quite time consuming to get it right.)
Flat surfaces with a different diffraction index have different Brewster angles (image):

Prossess
First, 'calibrate' it before measuring. Start the laser at 90$^o$ and make sure the reflected ray hits your laser; this will ensure that your protractor is parallel to the gem's reflective surface.

Then find the brewster angle. At the brewster angle the reflected ray is linearly polarized and its polarization axis is normal to the plane of incidence. Placing a polarizer with its axis normal to the polarization and changing the angle of the incident beam we can find the minimum intensity.
Make sure you don't rotate the pointer while moving it, since it usually produces partially polarized light.

Finally you can solve $\theta_B=arctan\frac{n_2}{n_1}$ for $n_2$. (Note that the angle the protractor shows  is not $\theta_B$; it's $90^o-\theta_B$.) There is a large error margin but you can easily distinguish ruby's 1.76* and CZ's 2.16.
* Refractive index at ~700nm (varies with wavelength, so you should use the corresponding index if you use a different wavelength).

None of the above methods can determine with absolute certainty whether it's a real ruby or not. Combined thought they can give you a very high confidence (excluding cases when the inside is not ruby, rather only the outside surfaces, if that's even possible). Personally I don't know any other material that has about 1.76 refractive index, hardness >8 and fluoresces red when hit with violet, other than ruby. Then again I am no expert.
Lastly, there are other methods like Raman spectroscopy, but assuming you don't own a 200,000$ spectrometer you would have to take your gem to a lab.
I've done all of the above tests, then took it to a lab. It turns out it was a real ruby.
