There are really only three ways into a watch. The crown (incidentally, this is where most of the problems evolve), the case back, and the crystal. The Crystal is a 'glass' covering the dial of the watch, protecting it from dirt, water, air, and the elements. There are three major types of crystals produced and used in watch making.
The best is sapphire, generally man-made sapphire. Sapphire is one of the hardest substances on earth, right up there with diamonds. Synthetic sapphire is a very hard, transparent material made of crystallizing aluminum oxide at very height temperatures. When it is heated, the synthetic sapphire forms round masses that are sliced into pieces with diamond-coated saws. These disks are then ground and polished into watch crystals. This is an exacting and demanding process since the end result ranks a nine on the hardness scale, with only diamonds being harder. Usually the sapphire crystal is made in a lab, but do not be fooled, the process does not stop here. The next step is an anti-reflective coating. This makes the sapphire crystal even more radiant and clear.
The benefits of sapphire crystal are numerous, and range from increased clarity to scratch resistance. The Smith & Bradley Sans-13 and the Gray Gaulding Limited Edition use anti-reflective Sapphire crystal. Mineral crystals are made of glass, and have many of the same attributes as sapphire crystal. One of the main benefits of mineral crystal is that it can be polished and buffed out if scratched. it is still very hard, with a 7 on the Mohs scale of hardness. These are less expensive as sapphire crystal, but by no means cheap. Additionally, these mineral crystals are easily replaced and repaired if damaged. The Smith & Bradley Atlantis uses a mineral crystal.
Acrylic is the most affordable type of crystal. It is totally synthetic, and has a plastic feel. It can be easily polished to remove light scuffs. Smith & Bradley does not use this type of low-price crystal. This crystal was used widely in vintage watches from the 1960's-1980's.