Sapphire glass used FOR Apple Watch: development, advantages and technical background

In today’s article, we take a detailed look at the use of sapphire glass in smartwatches, using the most popular smartwatch, the Apple Watch, as an example — from the first Series to the current Series 10. We compare the advantages and disadvantages of this high-end material against Ion-X and Gorilla Glass, explain the technical reasons why sapphire glass plays a central role in the Apple Watch’s health functions, and examine its scratch resistance based on independent tests. We also compare the Apple Watch with smartwatches from other manufacturers and provide an outlook on possible developments for the upcoming Series 11.

Development of Sapphire Glass Use from Apple Watch Series 1 to Series 10

Since the launch of the first Apple Watch in 2015, sapphire glass has established itself as a premium feature. As early as Series 0 and 1, Apple used sapphire glass for the back and the front on higher-end models (stainless steel and above). With Series 3 and subsequent versions, this practice was continued. Since then, sapphire glass has been used on the front of the stainless steel, titanium, and Hermès models, while the cheaper aluminum versions are equipped with Ion-X glass.
From Series 6 onwards, sapphire glass was further optimized to improve light transmission for enhanced sensors, which was especially important for health functions like blood oxygen measurement. Even in Series 10, sapphire glass remains a key feature in the higher-end models.

Advantages and Disadvantages of Sapphire Glass vs. Ion-X and Gorilla Glass

Advantages of Sapphire Glass:

• Scratch resistance: Sapphire glass is one of the hardest materials after diamond, making it significantly more resistant to scratches than Ion-X or Gorilla Glass. This makes it ideal for watches that are worn daily.
• Durability: Thanks to its high scratch resistance, the surface remains unscathed and clear even after years of use.
• Luxurious appearance: Sapphire glass has a smooth, glossy surface that provides a more premium look, complementing the high-end models of the Apple Watch.

Disadvantages of Sapphire Glass:

• Brittleness:Sapphire glass is less shock-resistant than Ion-X or Gorilla Glass and may break more easily upon impact.
• Cost:Sapphire glass is more expensive to produce, which is why it is only used in the premium models.
• Weight:Sapphire glass is heavier than Ion-X glass, making the aluminum models lighter and sportier overall.

Ion-X glass  and Gorilla Glass offer better shatter resistance and are cheaper to produce, but are much more prone to scratches.

Technical Background: Sapphire Glass and Health Sensors

The use of sapphire glass is not only aesthetic but also has technical reasons, especially in relation to the health sensors on the back of the Apple Watch. Sapphire glass offers high light transmission in the relevant spectral range, which is crucial for optical sensors measuring heart rate and blood oxygen levels.

Blood oxygen measurement: This function is based on photoplethysmography, where light is sent through the skin. The sensor measures how much light is absorbed by red blood cells. Different wavelengths – typically green, red, and infrared light – are sent through the tissue, and the difference in transmission is used to calculate blood oxygen saturation. Sapphire glass ensures undistorted light transmission, which improves measurement accuracy. Detailed information on the measurement principle can be found here: https://peterhcharlton.github.io/publication/wearable_ppg_chapter/

Scratch Resistance Tests: How Durable is Sapphire Glass?

Several independent tests, such as those by JerryRigEverything, confirm the exceptional scratch resistance of sapphire glass. In a well-known test, the glass is checked on the Mohs hardness scale. Sapphire glass only starts to show scratches at a hardness level of 8, whereas Gorilla Glass begins to scratch at level 6. These tests highlight the superior durability of sapphire glass compared to other materials.

Comparison to Other Smartwatches (Samsung, Fitbit, etc.)

In the premium segment, few manufacturers use sapphire glass for their smartwatches. Samsung mainly uses Gorilla Glass DX+ in its Galaxy Watches, which also offers high resistance but does not achieve the scratch resistance of sapphire glass. Fitbit generally uses Gorilla Glass 3 or 5 in its models, which is cheaper and more resistant to shattering but much more prone to scratching.
Garmin offers sapphire glass as an option on some of its high-end models, such as the Fenix series, similar to Apple with its premium models. This shows that sapphire glass is mainly used in higher-priced and sport-oriented models.

Outlook and Rumors for Apple Watch Series 11

Rumors suggest that Apple Watch Series 11 will continue to use sapphire glass for the premium models but with an improved coating that will make the glass even more impact-resistant. Additionally, Apple may introduce new health sensors that will benefit from the even better light transmission of sapphire glass. A potential feature being discussed in relation to sapphire glass is the integration of an optical sensor for blood glucose monitoring, where sapphire glass could play a key role due to its optical clarity.

Conclusion

Sapphire glass remains a key feature of the Apple Watch, especially for premium models. Its extreme scratch resistance and optical clarity make it the top choice for demanding users. While other manufacturers like Samsung and Fitbit rely on Gorilla Glass, sapphire glass is mainly used by Apple and Garmin in their premium models. With the Apple Watch Series 11, we could see further technological improvements that take full advantage of sapphire glass’s potential.