Tim Cook touted the durability of Sierra Blue in the September Apple Event.
Below is Apple press release where it is mention again.
“The all-new Pro lineup features a premium flat-edge design, crafted with exceptional materials, including the surgical-grade stainless steel band, with an elegant finish that is resistant to abrasion and corrosion, and a textured matte glass back. Both models are available in four striking colours including the all-new sierra blue, created using multiple layers of nanometer-scale metallic ceramics applied across the surface for a stunning and durable finish.”