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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:
enter image description here

This is the back side of the ruby:
enter image description here

Photo quality isn't that great. In reality, they are much prettier.

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    $\begingroup$ How is this question offtopic when the answers involve mostly optics? Please do elaborate. $\endgroup$ – Fermi paradox Jun 20 '18 at 19:48
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    $\begingroup$ Your solution to your question may involve optics but your question isn't conceptually about physics but rather about identifying gems which is more related to geology I guess. $\endgroup$ – nluigi Jun 20 '18 at 20:02
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    $\begingroup$ Should I phrase it as 'which physical properties of a ruby can I test at home'? (actually I just did) $\endgroup$ – Fermi paradox Jun 20 '18 at 20:22
  • $\begingroup$ @nluigi Actually you are right. I didn't know SE had a geology site. I'll discuss it with them and perhaps move for migration. Though it would be very annoying if their site (being beta) gets removed for whatever reason and the effort I put into this goes to waste. $\endgroup$ – Fermi paradox Jun 21 '18 at 5:09
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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):

cz_no_laser $\longrightarrow$ cz_with_laser

On the other hand when hitting the ruby with a violet laser it shines brightly in red:

ruby_no_laser $\longrightarrow$ enter image description here

(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):

brewster_angle_polarization

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.

enter image description here

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.

enter image description here

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 any other material that has about 1.76 refractive index, hardness >8 and fluorescences 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.

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  • $\begingroup$ There was an interesting phenomenon which I noticed but don't know why it happens. If someone can explain it, please do. When shining the ruby with linearly polarized white light, it had slightly different color based on the polarization axis. At the 2 extremes it was turning purple and orange respectively. $\endgroup$ – Fermi paradox Jun 21 '18 at 6:10
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    $\begingroup$ My guess is that this has to do with ruby being optically uniaxial crystal. $\endgroup$ – Jarosław Komar Jun 21 '18 at 12:30
  • $\begingroup$ Rubies are hexagonal so the optical properties will appear uniaxial when viewed along the c axis but be biaxial at other angles. Try viewing it with polarized light with another polarizer after the light exits and rotate the second one. $\endgroup$ – haresfur Jul 4 '18 at 22:06
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    $\begingroup$ Actually 405 nm is violet, not blue. Blue is around 450-470 nm. $\endgroup$ – Ruslan Nov 7 '18 at 16:16
  • $\begingroup$ @Ruslan You are right. It looked somewhat blue (the target surface was fluorescing) so I didn't give much thought to it and wrongly assumed it's blue. $\endgroup$ – Fermi paradox Aug 3 at 18:31

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