Again, this is Gorilla Glass vs Sapphire. I think that the Ion-X is Apple’s somewhat equivalent to the Gorilla Glass 3. The Gorilla Glass is also toughened by ion exchange , same process as the Ion-x. Apple mentioned this process in the Apple Watch product page.
The wonderful thing about plastic and glass crystals is the fact that they can be custom cut, shaped, or trimmed to fit different watch cases. Typically in the luxury watch field, sapphire crystals are preferred. Sapphire is extremely strong and scratch resistant – making it the top choice for a fine timepiece.
There is a video of one watch manufacturer dropping large ball bearings onto various watch crystals from a height of roughly 20". Results are interestingI'll try to find the video. In the meantime, there is THIS video of gorilla glass vs sapphire smartphone screens being tested, and demonstrating the brittle nature of sapphire.(Garmin Epix Pro Gen 2 Sapphire 51mm) Gorilla Glass is one of the most popular brands of chemically strengthened glass, manufactured by Corning. Several versions have been developed, the newer ones being more durable and providing better damage resistance.