I posted the following as a reply to someone who asked for information on Crystals a few posts down, and then figured why not post this by itself and expand on it so we can save it as a resource the next time the "what crystal is best" question pops up (and you know it will). Anyhow, please chime in on anything mentioned in my write-up and expand or clarify anything said. I will then compile all the information into a single article on crystals. BTW, I didn't say anything on glass crystals because I know little about their composition, and I intentionally said very little about acrylics.
A LITTLE INFO. ON WATCH CRYSTALS:
There are four main types of crystals that concern us here: Sapphire, Seiko Proprietary, Acrylic & glass.
Artificial sapphire (as is used for watch crystals) is grown in a crystal lattice, unlike the flame-formed sapphire, like the kind you find in common graduation rings. The Sapphire crystals used in the overwhelming majority of watch crystals are artificially grown (in a boule. Once formed, the boule is then sliced and the pieces are then cut and polished to the desired shape. This material is very hard an very resistant to surface scratches from common every day use. However, it is also (like diamonds) brittle and even though it is 4 times harder than Hardlex, it has a tendency to cleave or shatter on impact (provided the impact is just right). Sapphire crystals are also far more difficult to manufacture to high tolerances (read, to avoid micro fissures which could lead to weakspots on the crystal) which adds to their high price. Be warned, not just because a watch has a sapphire crystal does it mean it has a high quality piece suitable for difficult environments such as scuba diving. Flame formed sapphire while chemically identical, lacks the internal crystalline structure and is much softer and brittle. Don't forget that despite myths to the contrary all crystals can be scratched - sapphire crystals are just more resistant to surface scratches.
Hardlex is a Seiko proprietary type of hardened mineral crystal and comes in at least two different varieties (what goes in Seiko 5's is not the same quality of what goes into ISO divers). Hardlex is closer to 7 in the Moh's scale, but is much more flexible than sapphire. IOW's sapphire is harder but more brittle. Hardlex will scratch easier but resists impact much better. You can read more about Hardlex and the different types of it here:
Sapphlex is also a Seiko proprietary type of hardened mineral crystal that is laminated (layered on the outer side of the crystal) with sapphire. The idea being to provide the best of both sapphires's superior scratch resistance and Hardlex's superior impact resistance.
The types of plastics used to make acrylic crystals has varied widely throughout the years. From a performance POV there are acrylics out there which perform very well in professional divers (most dive computers today use acrylic crystals). The problems with acrylics are that although they can be made to be very very impact and preasure resistant (at least in higher end ones), they are highly susceptible to scratches from simple bumps that would normally not scratch a simple mineral crystal. Provided the scratches are not too deep, they can generally be easily buffed out with the appropriate tools.
Buffing out scratches on mineral crystals (of any kind - sapphire included) is possible, but difficult and time consuming. even then if you are able to remove the scratch, you run the risk of altering the shape of the crystal in that spot. There are no real guaranties as to the quality of results. Although it is possible, it is usually not worth the time and effort to repolish a mineral crystal. Also, since replacements are generally inexpensive, most people prefer to just replace them.
Personally, I have no problem with Seiko's choice of high-end Hardlex (it is not the same stuff that goes on Seiko 5's), as it has superior impact performance to Sapphire and is not that drastically softer than sapphire (7+ on the Mohs scale vs 9 for sapphire).
The overwhelming majority of mineral crystals (Sapphire, Hardlex, Sapphlex, or glass) come in of of two general shapes: flat or domed. There are tons of variations on the actual shapes of each from (and acrylics come in many more forms). There are pro's and con's to both general shapes.
Flat crystals tend to have a cleaner look and when used in tool watches such as divers, tend to be easier to protect, as they generally sit slightly lower than the bezel that surrounds them. The problems are that the flat shape makes them much more susceptible to impact failure (shattering and cracking), and the shape of the glass tends to act like a mirror when viewed at an angle - especially under water (this issue can be easily resolved with the use of AR coating, IMHO, preferably on the inside of the crystal).
Domed crystals, by their very nature distribute impact forces more evenly around the crystal and are thus more resistant to impact. Generally this characteristics tends to increase with the curvature of the dome. however, because of their raised profile dome crystals are far more susceptible to scratching, especially near the crystal's apex. the domed shape of the crystal naturally does away with the mirroring effect observed in flat crystals, but also distorts the image of the dial underneath it. Again the distortion is more pronounced the greater the curvature of the crystal. AR coating can be applied to domed crystals, but many would argue that it is not really necessary.