Crystal
The clear, rigid surface inside the bezel that looms over and defends the dial is called the crystal. Crystals vary in composition and as a result have particular attributes and disparate costs. While they can be flat or curved, the latter often increases the cost and desirability of a watch as it requires much time and effort to achieve the proper result. The difference between the various types of crystals will help you in accurately assessing a timepiece and determining whether it's the right choice for you.
Crystal Types:
Flame Fusion: Synthetic crystal created through a process that utilizes high heat, high pressure and aluminum oxide. This combination ensures the mineral (glass) and sapphire properties are fused together, resulting in the impact-resistance of a standard mineral crystal and offers the scratch-resistance of the sapphire.
Mineral: Technical term for glass
Sapphire: High-end, synthetic crystal that adds greater value to a timepiece. Sapphire is one of the most desirable crystals because it is transparent, very strong and scratch-resistant. In fact, the only natural substance able to scratch a sapphire crystal is a diamond.
Sapphire Coated Mineral: Combination crystal that capitalizes on the strengths of the two different substances
Proprietary Substances: Several watch manufacturers use their own particular materials for their crystals and assign them original names