For a variety of reasons, you may desire to have the crystal in your pocket watch replaced. It may be missing, broken, cracked or it may even have lost its transparency. Perhaps also, your crystal is "old era" plastic and, if so, for the reasons discussed below it should be replaced.
I am trained in the replacement of crystals and thus another service that I provide. I use both new old stock mineral glass and "new era" plastic crystals for this purpose and maintain an extensive crystal inventory for both Hunter and Open Face case styles. I also have an excellent inventory of the "American Bulls Eye" style for those early pocket watches and can provide prompt service for all. All that said, there are certain size crystals that are getting much harder to come by, thus turnaround for these may take a little longer and carry a premium cost to replace.

More information on crystals...
The piece of glass that covers the dial of your pocket watch is traditionally known as the "crystal". That doesn't mean it is made of lead crystal material, in fact it isn't, but  given its "clear transparency" has become known as such. The crystal material for original pocket watches was known in the trade as "mineral glass"and they were produced in great numbers. Such that, new old stock still remains available today. Mineral glass is ordinary glass that has been hardened and tempered to be more scratch resistent and less susceptible to breakage. 
There are two main case styles for American Pocket Watches, each requiring a different type of crystal. There is the Hunter Case a/k/a Geneva style case which has a hinged, metal cover (lid-style) over its crystal and dial and then there is the Open Face style case with no case-lid. For a few reasons, it costs more to replace a crystal for a Hunter Case than for a Open Face case. The Hunter Case crystal is much thinner and flatter so as to accommodate the lid and thus more fragile, more susceptible to breakage. Over the years, this trait has eroded their availability, causing those remaining to be higher cost. Crystals for Open Face pocket watches are generally thicker, less susceptible to breakage, and new old stock generally more readily available. It also takes more time to install a crystal for a Hunter Case. Typically, more care and attention is needed given the close tolerances one is working with. Perhaps also there is a dent in the metal lid that would need to be rubbed, pressed out, else it would crush the crystal when attempting to close the lid.  All this is considered in the pricing and effort to replace the crystal in your pocket watch.
With regards to plastic crystals, these never were original to antique American Pocket Watches and you may desire the original, period correct mineral glass. Perhaps a more important reason is that the early manufacturing process for plastic crystals was not perfected. Consequently, plastic crystals produced during this era contained unstable materials that emitted gasses. I term these "old era" plastic crystals. Over time, these gasses can cause oxidation in your movement and result in irreversible damage. Thus, they should be replaced else your family heirloom or treasured collector piece could become ruined.
Fortunately, the manufacturing process was eventually resolved and today high quality, stable and very clear plastic crystals are produced for both Open Face and Hunter Case crystals. I term these "new era" plastic crystals and provide them as part of my crystal replacement services as a quality but more economical alternative to mineral glass. 
A cautionary note, never really knows what era a plastic crystal for sale on Ebay or similar is from. Thus, one should only trust today's "new era" sources else you risk damage to your favorite timepiece. 
Contact Dave for all your Clock and Pocket Watch repair needs at 518-843-5809 or

American Pocket Watch Crystal Replacement

  Open Face Pocket Watch

Some my pocket watch crystal inventory.

My extensive inventory of "American" bulls eye crystals for those earlier pocket watches

 Hunter Case Pocket Watch