I don’t know for certain but let’s just say I won’t be rubbing my sapphire crystals on a wall anytime soon to find out.

 

. . . One individual in the thread said by the laws of physics only a . . . .

:-d

Yogi: Please save us!

 

Forgive me but what are we looking at here? It says worn pallet stone but I can not tell what the picture shows.

 

It is durable, I have a twenty year old Omega that is still has a perfect crystal. I also have a Tag with what would be a perfect crystal apart from the fact that a red setters claw dragged across the face and left a small permanent scratch, it is small and only noticeable when pointed out but it is there.
However, crystal is really the best, my watches with mineral glass all have a multitude of scratches chips and marks. The Seiko hardlex is better by far but is still marked albeit lightly.
So, I would say that you don't have to worry overly about crystal but if a dogs claw scratches it I suspect, as sticky says, a wall would do it no good!

 

A dog's claws are not nearly hard enough to mark sapphire. More likely, there was some hard grit embedded in your dogs claw, picked up while walking outdoors, that was able to scratch the sapphire. Not sure if Tag uses external AR coatings, but that coating layer is pretty easily scratched if it's there.

 

My wife has a Citizen Signature series watch that has a sapphire crystal. She can be pretty hard on watches so I figured the sapphire would be best. But after 2 years she did manage to scratch the crystal. I was pretty amazed that it was scratched as I too heard that nothing less than a diamond could scratch it. She does still wear her engagement ring so I guess it could have been that. I'll take a picture tonight of the scratch.

On the flip side, she just got an Omega SMP and it has a sapphire crystal. She wears it daily and for now takes it off for menial tasks. But I think that's cause it's new and cost more than any other watch she's worn. In time I'm sure her concern will fade and it will be kept on more often. Perhaps she'll manage to put a few scratches in that one too.

 

...and we’re off topic in 3.5 posts...

 

...and we're off topic in 3.5 posts...

I apologize to the OP & Pithy. . . .

No worries, knewb.

Some are still struggling to keep up after 10 years or even 10,000 posts.

 

I’ll jump in with some of my findings that I stated in that thread...

As far as Saphire Crystal, I have a Movado that I managed to put a small scratch in, believe me, it’s much easier to do than you think.

It was suggested in that thread that the scratch could possibly be in some type of AR coating. I am not aware that Movado uses AR coating on their Saphire crystals, but even if they do, the scratch is beyond that as I can feel it with my fingernail pretty good. As far as very few materials having the ability to scratch Saphire, this isn’t completely true. Believe you me, I had no intention of scratching my crystal yet the scratch is there.

Another theory is that the scratch is in fact a crack instead. It is certainly scratched, not cracked. So, either Movado is advertising in complete dishonesty about what their crystals are made out of, or the wall I banged it against was made out of something like diamonds, which I can assure was certainly not the case.

 

Thank you Pithy, I see it now.

 

I apologize to the OP & Pithy. I made a weak attempt at humor, and now see it in the photo too. I’m sorry.

Seriously though... Did the Mohs scale account for kinetic energy? That may explain the scratchability of sapphire, as it is scratchable by far less than diamond in the real world.

 

Anything can scratch the sapphire crystal if hit the "right" angle.... even diamond can be chipped
also, Silicon Carbide

 

Anything can scratch the sapphire crystal if hit the "right" angle.... even diamond can be chipped

What he said. There's a difference between a scratch and a chip (fracture). Talking about two different mechanisms.

Minerals have something called a cleavage angle (get your minds out of the gutter, boys...) which is the angle at which the mineral, if struck, will split along a predetermined plane and leave a nice clean flat surface. That's what the diamond setter does to get all those nice facets on diamonds...

 

Sapphire is very durable. It's used in the military aviation world as windows for sensors on supersonic aircraft. It can take the heat and abrasion flying through dust, rain, etc. That said, you CAN chip sapphire with something that's softer than diamonds. You can scratch sapphire with a very sharp point of reasonably hard things, because you're actually chipping out individual molecules of sapphire. Moissanite is harder than sapphire and softer than diamond. It's silicon carbide and it naturally occurs in meteorites, but it's rare to find it naturally. And there are man-made materials that are harder than sapphire but softer than diamonds (tungsten carbide, some ceramics (floor and bathroom tile), boron nitride, for example). The most common abrasive in sandpaper is aluminum oxide, which is the same as sapphire. You can scratch sapphire with sandpaper.

 

One individual in the thread said by the laws of physics only a diamond could scratch a sapphire crystal and

Velocity, repetition, or sheer bad luck can all allow a softer material to damage sapphire.

A rubber ball at 1000km/hour can crack concrete. Water can carve a valley out of rock given enough time. And being hard also makes sapphire more vulnerable to shattering, which can happen if you hit it just right.

So, no, sapphire is not invulnerable to something softer. But in practice, it's pretty difficult to scratch. Many complaints about scratched sapphire are actually about damage to an AR layer on top of the crystal, but not all of them.

 

Human fist softer than concrete tiles yet I could crack it with my fist when performing Tameshiwari.

 

thuper durable!!!

 

I’ve never scratched a sapphire crystal, but I’ve certainly seen marred AR coatings on sapphire crystals.

 

NASA asked the same question about how a softer material could damage a harder material.

 

According to the guy that caused this thread to be made, NASA are probably flat earthers because there is no way foam can break RCC. They must not understand the laws of physics, and that RCC must be not actually RCC but something else like paper or something. Or maybe the foam was actually made of diamond?

The law of physics apply regardless of anyone's beliefs. If you scratched a watch crystal with a brick, then what you scratched is not sapphire. It is that simple.

That being said, we live in a world where there are international conventions for people who belief that the Earth is flat and US congressmen think that the sea level is rising because of rocks falling from cliff faces into the sea. I can certainly understand someone choosing not to believe in how material hardness works, and you won't have any more comments from me regarding this.

I'm very sorry to hear that you scratched your sapphire, but I'm talking about the law of physics, not hearsay. The Mohs scale is very simple actually: "The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, or the softest material that can scratch the given material" (wikipedia).

So if you indeed managed to scratch your sapphire crystal by banging it on the wall, either the crystal isn't sapphire, or your walls are made of diamond.

 

Now this is a topic I like!

Scratch resistant, strength, and impact resistance are very different things.

Scratch resistance is best assessed by the Mohrs harness number (note the term 'hardness') and sapphire at 9 is indeed only surpassed by diamond at 10. It's matched by quite a few other materials though, some of which can be 9 and a bit. Tungsten carbide for example that machine tool bits are made from.

Strength is a measure of how much force is required to break something. From memory, sapphire is about 2 GPa in compression and 0.6 GPa in tension. It's like concrete in that it's stronger in compression than tension. Take a thin piece of sapphire, such as a watch glass, and push on the centre and the stresses in it become compressive on the side of your thumb and tensile on the opposite face. Thin piece of sapphire, hard push with a thumb and it'll snap. Don't try though because it's damned sharp when it has snapped.

Impact resistance is a measure of how much energy the material can absorb before it fractures. A really unfortunate fact about materials is that as hardness (Mohrs number) goes up, impact resistance comes down. This is best shown in the carbon content of steels, where low carbon steels can deform a lot and absorb a lot of energy before they fracture, but higher strength high carbon steels can't deflect as much and so can't take as much impact energy.

So, in watch terms, sapphire is more scratch resistant than chemical glass, stronger, but less able to take impacts. Just ask the number of watch owners who've shattered a sapphire crystal by knocking it on a door frame or dropping their watch onto a tiled floor. Also, and very handy with watches, sapphire is much more transparent than glass and has a lower refractive index. This basically means it's easier to see through and particularly good if you look through it at an angle rather than face on.

There are ways to limit the weaknesses of sapphire in watch glasses, but they're all in the design and manufacturing stage and need a bit of engineering knowledge, which some watch companies don''t really possess.

 

Good breakdown. In the real world, these are all interrelated, making something that seems black and white various shades of gray. That the problem with people stating absolutes. There are very few absolutes in the real world.

 

Never seen so much waffle, it’s durable yes, extremely!

 

Never seen so much waffle, it's durable yes, extremely!

But if you take a toasted waffle and throw it fast enough at a window it'll break the window. The waffle isn't as hard as glass, but it'll still shatter it.

I agree though that sapphire is very durable. Good watch design ensures the material constraints are accounted for and the chance of exceeding them is minimised

 

Can sapphire scratch sapphire? Say, two watches being carried unprotected?


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Can sapphire scratch sapphire? Say, two watches being carried unprotected?

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Yes, but unlikely as the sapphire on either watch will be flat or gently curved, so no sharp point to scratch with

 

Not sure if mentioned but they have anti reflective coating which often is far easier to scratch and looks like the sapphire scratched.

 

Different forms of Corundum can be found in rocks. It is a very hard natural mineral. Which is basically sapphire. Depending on the type of brick made, it can have various abrasive materials in it. Corundum wouldn't be out of the realm of possibly to be in certain types of concrete or brick. Finely ground up and dispersed.

A dog's toenail is made from keratin. The same as human nails and our hair. It is extremely unlikely that would, in and of itself, be enough to scratch a sapphire crystal.

 

Remember,sapphire can still scratch another sapphire as both have the same hardness (provided that there's a sharp point on either one)

I have a watch with domed sapphire that I wear often,some parts of the bezel are scratched but the sapphire is still spotless

I have another watch with sapphire crystal that was marked because I accidentally hit the edge of the glass table so it's glass vs sapphire but there's a small white mark on the sapphire

 

Moh's scale involves rubbing two samples against each other, and seeing which one has a visible scratch. It doesn't not involve applying a substantial impact, that can cause a very scratch resistant material to chip. I have chipped a sapphire crystal by hitting it against a metal railing.

 

Moh's scale involves rubbing two samples against each other, and seeing which one has a visible scratch.

I'm going to ask my wife whether she's interested in conducting a Moh's test tonight.

Yeah, it's juvenile but couldn't resist... :-d