The 'crystal' is the transparent cover protecting the dial. It is not literal crystal — the word just refers to whichever clear material is used, and there are three common types.
Cheapest and used on many vintage and budget watches. It scratches easily, but light scratches polish right out with a little paste, and it is almost impossible to shatter. It gives a warm, slightly domed vintage look.
Ordinary hardened glass, common on mid-range watches. More scratch-resistant than acrylic, but if struck hard it can crack or chip rather than scuff.
Synthetic sapphire is extremely hard — second only to diamond — so it resists scratches better than anything else and stays clear for decades. It is the premium choice, often with an anti-reflective coating. Its one weakness: a very hard knock on a sharp edge can shatter it.