The watch crystal serves a very important purpose. Beyond just providing a clear perspective of the dial, it also protects the dial and hands from the damage and can be an important design element by including facets, bevels or magnifying windows.
Crystals come in three basic materials... plastic, mineral glass, and synthetic sapphire. Plastic crystals are inexpensive and easy to install. They're easily scratched but it's also easy to buff out surface scratches. Mineral glass crystals are slightly more expensive and more difficult to install. They are harder to scratch but once scratched they can't be buffed out. Sapphire crystals are much more expensive and may require specialized equipment to install. But they are very hard to scratch - which is why they are used on most high-end modern watches.
Crystals on vintage watches are easy do-it-yourself projects if you have an interest to tackle it.
Take for example this 1931 Hamilton Perry. Not only does it need a new crystal, it also needs a new number 1 hour marker (we'll save that for another day).
Looking through the old crystal offers an even better perspective.
So your first question is probably, "how do you get the old crystal out?". Sometimes it's already loose and you can just push it out with your thumb. But most of the time they are not loose.
Non-round crystals are typically glued into the bezel with either a "cement" or "UV-curable glue".
Round crystals are often friction fit (principally the acrylic / plastic kind) where the diameter of the crystal is ever so slightly larger than the bezel opening. A special tool is used to remove and install those crystals.
For a non-round crystal like on this Perry, it's glued in and if it's not loose you can free it by either (a) boiling it in water for a minute or (b) put it in an ultrasonic jewelry cleaner for a couple of minutes. Both ways will do the trick.
Once it's out, you can get down to business. The next step it to make sure to remove any left over glue in the bezel opening by using a sharp tooth pick. Avoid using anything metal to scrape out the glue - as one small slip will scratch the bezel and you don't want that.
I find the best way to shape crystals is with 600 grit sandpaper - available at most hardware stores. Plastic crystals are easiest to shape but glass crystals are shaped this way too - it just takes a lot longer. All you need to do is put a few drops of water on the paper and work the crystal back and forth along the edges.
Take your time and work each edge until the crystal fits nicely into the bezel opening.
Now you're ready to glue the crystal in. The type of glue really depends upon the type of crystal. As a general rule, plastic crystals are installed with crystal cement and glass crystals are installed with UV glue. I think both will work on either but they're best used if applied like above.
UV glue will cure in the sunlight, if your wondering where to find UV rays - although there are inexpensive lamps that ladies use to cure fingernail glue that you can use too.
Cement smells a lot like the glue I used as a kid to make model airplanes and cars... if you did the same you know what I'm talking about.
For this crystal I used cement...
You need a steady hand and definitely DO NOT ATTEMPT THIS WITH THE BEZEL ON THE WATCH. It's an absolute certainty that you'll get the glue on the dial and that's not a good idea.
A fine bead is all you need and you can spread it more with a toothpick if needed.
Once you insert the crystal the glue will probably squeeze out. You can clean it up with a soft cloth and rubbing alcohol.
If you make a mess of it don't despair. Just take it all apart, clean it in soapy water, dry it and start over. You'll get the hang of it eventually.
Once it's all dried you can put the bezel back on the watch.
Now you can compare the before and after.
Changing crystals is an easy and rewarding project. It's definitely something you should be willing to try now that you know how to do it!
Now to find a number 1....