Stainless steel

Stainless steel is the material used to produce the vast majority of quality watches in the world. Much like with the 18-karat gold alloy, Apple is playing it safe and not using some highly proprietary alloy or cutting-edge process (such as their tie-up with Liquidmetal). Instead, Apple is working within the confines of the ASM 316L standard — a material more commonly (and incorrectly) known as "surgical grade stainless." Buzzwords aside, 316L stainless is often used in the production of medical instruments and implants, as well as food processing equipment. The reason is simple; 316L does a good job of resisting the seepage of metal particles from the finished component.

This makes it the most common stainless alloy used in the watch industry for a similar reason: nickel allergies. Watch manufacturers have decades of experience working with stainless steel to narrow its effects on those with nickel allergies, and companies such as Rolex and Omega have tuned their alloys and processes to the point where only the most acute sufferers would notice even mild effects. I suspect a large focus of Apple's metallurgy and process design was on the nickel allergy issue. Like the "Gold" video, "Stainless" opens with numerous beauty shots directly from the foundry floor as the molten 316L is processed into what foundries call "sticks." The molten metal passes out of the bottom of a crucible (typically located on the top floor of a multi-story foundry) through a valve and into a series of shaping steps that form each stick as the metal's viscosity rises in the transformation back into a solid. This process is precisely controlled in order to properly form the final stick's grain structure and hardness. Apple is producing the watches in enough volume that it can easily specify the exact alloy composition of the entire crucible of material, as well as define the precise temper, hardness and stick dimensions.