VentureBeat heard rumors that Apple originally planned to use sapphire-based screens for its iPhone 6 series but apparently the plan changed just weeks before the product's launch because the sapphire repeatedly kept cracking during standard drop tests. This urged Apple to switch to its second choice: "ion-strengthened" glass.
Apple's Watch will use sapphire for its screen but its believed that the smaller 1.5" and 1.7" watch screens were much less likely to break in drop tests.
IDC analyst Danielle Levitas says it isn’t clear when exactly the drop-test failures took place, or when Apple abandoned plans for sapphire-screened iPhones. She says the poor drop-test results, combined with the relative high cost of sapphire glass, could have made plans to ship sapphire glass phones too risky.
One researcher who covers GT Advanced Technologies, the company that was to produce the glass for the iPhone 6, wrote in a research note earlier this week that plans for the sapphire screens were cancelled in August, just weeks before the September 9 launch.