The transparent cover over the watch face is often called the crystal. That's because it is usually made of acrylic crystal, mineral crystal, or synthetic sapphire. In some cases, a watch cover may be made from a combination of these materials.
Synthetic sapphire is generally the most expensive, desirable, and durable of these materials. It's approximately three times harder than mineral crystal and 20 times harder than acrylic crystal. This makes it highly resistant to scratching and therefore an ideal choice to protect your watch dial and movement. But because it is so hard, it is also brittle, and shatters more easily than mineral glass or acrylic.
Synthetic sapphire is made by crystallizing aluminum oxide at very high temperatures. It is chemically identical to natural sapphire and one of the hardest substances in the world. Only diamonds and a few man-made materials that use silicon carbide are harder.