My best guess is that the eyes are emeralds. I have bought a few hundred emeralds in my time, as well as a few tourmalines, so that's my opinion. Can't be sure without personal examination though. However, if Tiffany makes a piece out of 18 ct gold, they are far more likely to use emeralds instead of tourmalines.
I did find a *somewhat* similar owl, which retails for $2,800, and actually sold for $925. However, it was only 14K, and not as well made as your piece. Here's the link: https://www.therealreal.com/products/jewelry/brooches/pin/brooch-14k-emerald-owl-brooch
Tiffany HAS had some of its pieces made in Italy, so that's helpful in deciding your piece is not a counterfeit.
Your piece is probably somewhat vintage, because the current Tiffany hallmark is usually a simple T & Co. Tiffany also currently uses 750 instead of 18K or 18ct.
To be really sure, you should test the gold in as inconspicuous place as possible--because the affordable gold tests involve acid. You can buy them on ebay and many other places. Or, you could pay for a laboratory to use a non-desturctive test like X-Ray fluorescence (XRF) spectrometry. If you live near a good university with a good chemisty department, they might be able to do that much more cheaply than a commercial lab.
Some Tiffany stores will evaluate a piece for free.
So, you might have a valuable Tiffany heirloom piece--hopefully. Good luck!
