A sapphire watch crystal is a watch screen that is fabricated using sapphire crystal. Sapphires are used due to their resistance and toughness as well as their aesthetic appearance.
Sapphire is the second strongest jewel on the Mohs mineral hardness scale, second only to the diamond. The crystal is usually made up of a sapphire-quartz combination. The screens are not completely shatter proof since strength is different than hardness, but it does mean that almost nothing other than diamond is able to scratch the face. This means that those who like to keep their watch faces completely scratch-free often go with this watch crystal.
There are many modern examples of using sapphire crystals as screens in watches and other devices. The Richard Mille watch series, including the Tourbillion, is one such example. Sapphire is tough, but it takes delicate work during machining. Such watches often take 1,000 hours of machining with up to 430 hours pre-forming case components and another 350 just for watch case polishing. Watches with sapphire crystals can often cost between $100 and $300 or more depending on the watch. They are also a common addition to diving watches due to their clearness and toughness despite water pressure of 50 meters or more.