Glass nail files, also called crystal nail files, are the perfect choice for those wanting to find an alternative to disposable paper emery boards. The usual zero waste suggestion for replacing paper emery boards is a metal file, but I have found that metal files are too coarse for fingernails.
Glass nail files work as well as emery boards, they are washable, and should last a lifetime if they are not accidentally broken. Don't get the idea that they are fragile, though. They are pretty sturdy, but they are glass, so it is possible to break them.
The only other downside to glass nail files is that most of them come in a plastic case. The one I bought came in a plastic case, but I didn't realize it came in one.
I did a little research to see if I could find a high quality company that sold these files and it looks like Zamberg or Soligen is the way to go. They are both German companies.
I like this file from Zamberg and this one from Soligen/Three Swords.
Another option is Etsy, where you can get something really unique.
