Normally they are along the girdle of the diamond.  You will often find a diamond with a couple of naturals on opposite sides of the diamond, it is a sign that the cutter took the minimum amount of size off the diamond.

Here is a trigon, on a diamond facet – but not along the girdle edge.  This is a much more rare type of natural, trigons do form on a lot of diamonds but they are not usually left on a polished stone.

What the heck is that?  Girdle Bearding is small feathers going into the diamond from the girdle.  They are not natural, they are caused by poor workmanship when cutting the diamond (no exceptions, it is not caused by wear or damage after cutting).

When the diamond is bruted (given its shape), if the guy doing the work is pressing too hard or trying to  get the job done too fast you can end up with bearding on the girdle.

The harder they push to shape the diamond the more bearding that can occur.  Some diamonds have a small amount of bearding and it would never be mentioned on a report (some is acceptable) but when it gets bad as shown in the photo here it will most certainly be mentioned.