Sapphire glass is widely used in wristwatches, professional chronographs and high-end smartwatches due to its hardness, excellent optical transmittance and chemical stability. Despite their chemical composition being identical to that of natural sapphire, the watch crystals are all made of industrially controllable, grown synthetic sapphire to ensure batch consistency and optical purity.

1. Optical property

(1) High transparency and broad-spectrum transmittance

The high level of transparency guarantees optimal clarity and legibility of the sapphire glass under all lighting conditions. This is accompanied by true colour reproduction and the assurance of there being no yellowing over a prolonged period of use. Sapphire glass, when combined with an anti-reflective (AR) coating, provides a superior visual effect to mineral glass.

(2) Refractive index and optical appearance

The refractive index of sapphire is approximately 1.76 to 1.77. The higher refractive index brings stronger surface reflection, which endows the watches with a unique luster and visual depth. However, it should be noted that excessive reflection under strong light may affect the reading. Therefore, the application of AR coating and multiple dielectric films on one or both sides is often used to effectively reduce reflection and enhance light transmittance.

(3) Low dispersion and color stability

Sapphire is characterized by low dispersion, minimal colour difference, and neutral colour reproduction. It is able to present the dial colour and pointer details reliably under both sunlight and artificial light sources, which is of particular importance for luxury watches that emphasize visual accuracy.

2. Mechanical property

(1) High hardness and scratch resistance

With a Mohs hardness rating of 9, sapphire is well-suited to resisting scratches from regular wear and tear, as well as metal contacts. This makes it an ideal material for daily wristwatch use. High hardness is associated with low toughness. While sapphire is resistant to scratching, its ability to withstand impacts is less robust than that demonstrated by mineral glass or acrylic. Severe impacts may cause breakage.

(2) Heat stability

Its low thermal expansion and high resistance to thermal shock ensure its reliability in diving, aviation and professional outdoor watches, and its ability to adapt to drastic temperature changes is also noteworthy.

3. Chemical stability

Sapphire displays robust chemical inertness and is resistant to water vapour, salt spray, weak acids, weak bases and most household chemicals. It will not experience surface corrosion, fogging or decline in optical performance. Sapphire’s key advantage over acrylic crystal made of organic materials is its superior resistance to aging, allowing it to retain its brand-new appearance over a prolonged period.

4. Manufacturing technique

(1) Crystal growth

This method involves melting high-purity alumina powder at high temperatures and introducing seed crystals. These crystals are then allowed to grow into large single crystals. The ingots are then cut, ground, polished, coated and tested, and it is possible to produce large-sized ingots with high optical quality and low internal stress.

(2) Main processing

Crystal ingot→Diamond cutting→Grinding and shaping→Edge chamfering→Ultrasonic cleaning→Optical inspection and thickness measurement→Coating

5. Common surface treatment techniques

(1) Anti-reflective (AR) coating

It effectively reduces glare and enhances contrast, and is commonly found in pilot watches, diving watches and dress watches.

(2) Oleophobic coating

It is resistant to fingerprinting, effortless to clean, and offers an unruffled surface texture.

6. Value of sapphire glass in contemporary watchmaking

The value of sapphire glass extends far beyond its scratch resistance. It combines optical properties, durability, high-end texture and environmental adaptability. Despite being worn for decades, it retains its original clarity and beauty. In the contemporary watchmaking industry, sapphire glass has transitioned from an optional configuration to an essential element.

Synthetic sapphire glass boasts a range of outstanding properties including physical, chemical and optical properties, as well as reliable large-scale production processes. These ensure the glass meets the high standards of durability and aesthetics required for use in high-end watches. Despite its high processing cost and limited impact resistance, its overall performance significantly exceeds that of mineral glass and acrylic, making it the core material in today’s watchmaking industry.