What makes the comparison between sapphire and Ion-X interesting is that it illustrates a classic Hobsian choice for engineers when looking to apply a material to solve a problem. Sapphire is very hard, making it resistant to scratches, but somewhat brittle. Ion-X is a more yielding material, able to withstand bending and impacts sapphire would crumble under, but it's softness also makes it more susceptible to surface damage.
For now, it appears the division of applying Ion-X to the Sport, and sapphire to the Watch and Edition models seems appropriate. Sapphire is the absolute standard crystal material for higher-end watches. It's had decades of service and fractures and chips have been relatively rare. Ion-X will likely show some scratches here and there, but is far more suitable to the athletic intentions of the Sport model.
In the future, the tradeoff between Ion-X and sapphire will become more interesting. Corning (and others) have very high demand for ion exchange hardened glass, and they are continually improving the production process and technology. Sapphire, on the other hand, has very limited opportunity for improvement. It will be interesting to see if glass can one day surpass sapphire across both scratch resistance and overall strength — it is entirely possible.
But Science: stainless steel is not a super-hard material. The 316L grade used in Apple Watch is actually softer than the 7000 series aluminum used in Apple Watch Sport. If you’re curious as to where 316L is positioned within grades of stainless steel, check out this helpful chart. Apple could have chosen to go with more durable 904L stainless steel, but it’s much more expensive to manufacture and would raise Apple Watch’s price. With a polished finish, even 904L can still be scratched fairly easily.
First, Apple has chosen to use a harder aluminum alloy — 7000 series. While the standard 6000 series aluminum is plenty strong for a watch, 7000 series aluminum offers roughly double the hardness and twice the tensile strength, approaching figures typically seen in mild steel. This means the anodized surface will have a much harder underlying structure to resist scratches and dents.
More importantly, Apple has deployed some advanced anodizing technologies. ...
The best theory that we've developed is that Apple is using a combination of tricks to achieve anodizing superior to the typical processes everyone else employs, combined with post-anodizing processing. Most likely, Apple uses the high currents and chilled baths of Type III anodizing to produce a dense, thin layer.
Whatever Apple is doing, I found myself impressed. I was prepared to write this piece with the conclusion that the Sport watches would be "essentially disposable" in the wear and tear department, but I made a field trip to all 3 Apple Stores in the Portland area. With the Watch Try On tables empty, the employees were happy to show me all 32 Space Gray Sport watches Apple has in the Portland area, and they were all still flawless. It's important to remember, these watches have been on-duty now for 10 days, being handled by literally hundreds of people. Even without rough handling, I would expect the anodizing to just start showing signs of wear on the edges, but every watch I inspected still had a flawless finish.
Second, the sapphire crystal used on “premium” Apple Watches almost doubles the ambient light they reflect, creating brighter screen and mirror reflections, color shifting, and reduced contrast, all of which are better with the Apple Watch Sport’s Ion-X glass.