Sapphire Crystal

Specification

Technical parameters

Overview

Synthetic sapphire is crystalline aluminium oxide — the same material as natural corundum, grown in a laboratory. It is used for watch crystals because it is extremely hard, around 9 on the Mohs scale, so it resists the abrasion that fogs acrylic and mineral glass within a year or two.

Hardness is not toughness. Sapphire is brittle, and a point impact that would only mark an acrylic crystal can shatter a sapphire one. It is also prone to chipping at the edge, which is why many designs use a slightly domed or box-shaped section and a polished rather than sharp edge.

Anti-reflective coating is the visible variable. A bare sapphire crystal reflects around 8 % of incident light, which is what makes an uncoated crystal look like a mirror in bright conditions. A multi-layer AR coating cuts that to near zero on the outer face — but the coating is far softer than the sapphire underneath and scratches readily, which is why some designs apply it only to the inner face.

What matters about it

• Extremely scratch resistant in normal wear
• Brittle — fails by chipping rather than marking
• AR coating dramatically improves legibility
• Coating is soft and can scratch even when the crystal does not
• Available flat, domed and box-shaped

Practical notes

• Press crystals with a flat die and even axial force only.
• Check the gasket seat before fitting — a chip in the seat will crack the crystal under pressure.
• An AR-coated crystal shows its coating damage as a purple smear in bright light.
• Replace the crystal gasket whenever a crystal is removed.

Compatible with