The crystal is the clear material covering the dial of a watch. Most often circular, they come in a variety of different shapes and materials.
- Flat - just like you think, the simplest and least expensive to manufacture
- Planar-convex - flat on the bottom, curved (domed) on the top
- Convex - Curved top and bottom, my personal preference
- Plastic - cheapest, can be quite resistant to shattering, easy to mold into any desired shape. The least hard, so easiest to scratch. Often possible to buff out scratches using toothpaste or car wax. Hardlex, Hesalite and other trade names. Plastic is popular among vintage fans, as that's where it saw a lot of use, but personally I avoid it.
- Glass or mineral crystal. Different names for varying formulas of glass. Varies widely in clarity, toughness and scratch resistance. Borosilicate versions can be quite good, for example Seiko SBDX001 'Marinemaster'.
- Sapphlex - A Seiko-only trick, sapphlex is glass with a layer of sapphire. Rarely seen.
- Sapphire - the very best. I wrote an entire page about it; go read!
- AR coatings for information on anti-reflective coatings for crystals