I might as well write something about it, because I just made one. Polished graphite edge prevents dust from settling on top of the edge or entering the book. It is very simple to make too, with just some graphite powder and sizing such as gelatin or paste. Some people use paste and graphite mixture, Gianfranco Mombelli recommends the Swiss gilding agent Auroschnitt, and I usually make my edges with gelatin dissolved in water. There are many ways to do it. I have come to understand that graphite edges are typical for Swiss bookbinding, and it is quite hard to find any writings or examples of them anywhere else.
In preparation, the top edge of the book is ploughed as smooth as possible. The other option is to cut it in a guillotine and then most likely spend the best part of the day sanding. The book must sit tight in a press so that you don’t get graphite or liquid inside the pages; If the paper in the book is glossy you might want to spread a thin layer of paste on the edge and let it dry before proceeding.
The graphite powder is rubbed on the edge with a finger or cotton cloth that has been moistened in the size of your choice. This is continued until the edge is fully covered and acquires a nice sheen. Some people prefer to make a mixture which is painted on the edge with a brush: this is a good idea if you’re interested in your health and don’t want to smudge graphite everywhere. Breathing the graphite powder should also be avoided because it will lodge in lungs.
Agate burnisher is used to polish the edge to a mirror sheen. The edge is usually waxed lightly with beeswax or microcrystalline wax to form a coating and to help with burnishing. Some binders finish the edge with a thin layer of fixative spray, but often this is not necessary. The finished edge can be altered by gauffering it with decorative tools. If the edge has been made on loose signatures, they can be shuffled with gilt or painted signatures to form a book with a striped edge. I have attached two additional pictures of an older binding to show a finished edge with gauffering.