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 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 only new old stock mineral glass for this purpose and maintain an extensive inventory of new old stock crystal sizes in both hunter and open face style. 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 Antique & Vintage American Pocket Watches, each requiring a different type of crystal. There is the hunter style 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 hunter style case than for a open style case. The hunter style 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 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 style 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 were never original to Antique & Vintage 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 replacment crystals was not perfected. Consequently, plastic crystals produced during this era contained unstable materials that could emit gasses. Over time, these gasses can cause oxidation to occur in your movement and result in irreversible damage. While the manufacturing process was eventually resolved, one never really knows what era a replacement plastic crystal is from. Thus, they should be replaced with new old stock mineral glass else your family heirloom or treasured collector piece could become ruined. 

American Pocket Watch Crystal Replacement

Some of my inventory of pocket watch crytals

Some more of my inventory of pocket watch crystals

My extensive inventory of "American" bulls eye crystals for those earlier pocket watches
A closer view of the "American" bullseye style of crytal. A flat spot is molded into the top. These were briefly popular in the early 1900's and are sought after today by collectors seeking originality. It appears they were modelled after a similar style popular in the early 1800's for Engish fusee watches.