Most times when you are talking about a newel post you are talking about a wood newel posts. There are some metal ones out there but most people prefer the look and feel of wood. There are thousands of species of wood but usually 18 or less are sold by a supplier. You will get the look and feel with any of these 18 you are wanting. Some cultures will have different types of wood that they will use.
The posts are one of the most important parts of your stairs. They act as an anchor for the handrail. If you have a hand rail that shakes you could be in for trouble. If it were to become too unsturdy someone could fall and have an accident. So that is why your posts are so important. If you have attached them and they are solid you don’t need to worry about them holding up over the years.