Blued screw

A blued screw is a screw that's been tempered at about 300 °C to toughen it. This incidentally turns it blue as a side effect.^[1]

Downmarket watch factories sometimes simply chemically blue or even paint their screws.^[1] This is much easier to accomplish, and has a similar visual appeal, but without the functional benefit.

Contrast with even the cheapest watches sporting real synthetic rubies, which are much easier to make, complete with their benefitial hardness.