Top part of the watch case, covering dial and hands is a crystal. It is usually made from one of the three major materials: scratch-resistant synthetic sapphire(best), hardened mineral glass(better) and acrylic(good). Some crystals can be made of two different materials for economy or durability. Some have date magnifiers attached to the top, like Rolex cyclop on a datejust model, or inside the crystal, like Muhle SAR model. Sometimes crystals are treated with a special anti-glare film on top and/or bottom.
Acrylic Crystal: Made of basic optical quality plastic. Very easy to scratch, but just as easy to polish off the scratches. Hard to break.
Mineral Crystal: Made of glass (silicone). Has a hardness of 600-800 vickers, may be scratched or cracked, but less likely to shatter as compared to sapphire. Very hard to polish, but possible.
Sapphire Crystal: Made of synthetic sapphire (aluminum oxide) and has a hardness of 2000 vickers, they are much more scratch-resistant than mineral, but may chip or shatter, if struck sharply and from the correct angle.