Watch crystal: sapphire or mineral glass?

Is there a definitive test for sapphire crystal?

We are often asked how one can tell the difference between sapphire crystal and mineral glass. Various 'internet experts' offer a variety of crystal tests - from tapping, heating, touching - even licking (yuck!)- to what is known as 'the definite' test: scratching the watch crystal with a hard and sharpen nail.

Of course, there is a simple and PROPER way to tell what the crystal is made of: with the help of a diamond tester, the result will be known in a second. A hand-held diamond tester checks the thermal conductivity and the result is displayed on a scale.

Bad news: most likely, you won't have one in your tool box, but the good news is: your local jeweller most likely will. The diamond tester is commonly used in the jewellery trade. If you ask your jeweller to perform a crystal test, he would surely be happy to help. (While there, place an order for a 3Ct diamond ring, that would make his day :)

The test is a straight forward one. Like natural sapphire, a synthetic sapphire watch crystal will 'excite' the LED meter to the very last red bar, just under the "diamond" level. Mineral glass will result in no lit LED lights. And it is as simple as that!

Contrary to commonly held opinion, that even a hard synthetic sapphire crystal like the one fitted on a Rolex watch CAN be scratched. If you are still tempted to go for a sharp nail test, do it in the privacy of your home. There is nothing more embarrassing than scratching your brand new Submariner in front of a dozen of your office mates. Unless you want to be remembered as "the watch guy".

Photos below show the reading on sapphire crystal front and mineral glass case back. The last one is diamond test.

Plexi vs. Crystal?

Here is a photo of Rolex Submariner 16610 that was dropped on a tiled floor. Due to the impact, the crystal glass cracked. Number of small glass particles went through date display window and found their way into the movement.

One not-so-small bit was lodged between sweep seconds wheel and centre wheel bridge which caused the watch to stop.

The owner was lucky that apart from a bent escape wheel pivot, no other damage was found. To my surprise, the balance wheel and escape wheel shock absorbers were not 'activated'.

Conclusion: If you crack the crystal, complete overhaul is MUST - even if you have a brand new watch or one that has been recently overhauled. For that reason watch fitted with plexyglass would be more useful under tough conditions (military or sport activity).

[Q]: Can I trouble you to explain or point me to some info on how the shock absorbers are supposed to activate?

In electrical circuit, the circuit breaker's job is to cut off the circuit whenever the current jumps above a safe level.
In mechanical watches, an equivalent of circuit breaker is shock protection system. The shock protection system is there to protect the critical alignment of components in the event of an unexpected physical shock, such as being dropped.
The pivots and jewels of the balance wheel are fragile in comparison to the mass they need to support, and without shock protection are the most likely part of the watch to be damaged under impact.

The Incabloc (or Kif in Rolex) system uses a specially shaped spring to allow the delicate jewels and pivots to shift in their settings under impact . When the impact is over, the springs guide the parts back to their original positions. If the impact is excessive, the shock protection system is activated and it needs to be reset by watch repairer.
Unfortunately shock protecting system is not perfect and it's efficiency and ability to protect vital parts is not always all that great. Angle of impact, G-force and surface play their role. In some cases, a relatively small drop could cause major damage and in other substantial drops cause very little or no visible damage.

[Q]: What is the difference between plexi glass, mineral glass and sapphire crystal?

Plexiglass is a clear, lightweight type of plastic. Mineral glass is ordinarily glass hardened by a tempering process. Synthetic sapphire crystal is a very hard, transparent glass made of crystallizing aluminium oxide at very high temperatures. When it is heated, the synthetic sapphire forms round masses that are sliced into pieces with diamond-coated saws. These disks are then ground and polished into watch crystals.

Plexiglass, as one would expect, is the least expensive. It is also the least likely to shatter and the most likely to become scratched. Mineral glass, even though it has been hardened by a tempering process, is more likely to break than plexiglass. But it is also more scratch-resistant than plastic plexy. Synthetic sapphire is the most expensive glass crystal and the most scratch resistant. Watch crystals made of synthetic sapphire are often marketed as "scratch resistant", meaning they are very difficult - but not impossible - to scratch. Diamond can scratch them; so can man-made materials that incorporate silicon carbide. As pointed out by other forum members, due to the hardness of sapphire crystal, it is also brittle and shatters more easily than mineral glass or plexiglass.

Unlike plexy, mineral and sapphire crystals can not be re-polished.

Which crystal is fitted to your Rolex watch? An easy way to tell this is by model reference number: in general, watches with 4 code reference number (like 5513, 1680 etc) are fitted with plexy and 5 digits (16233, 16610 etc) are fitted with sapphire crystal.