We don't yet know what our new friends are like, and our best friends are those we have known for a long time. I am looking for a phrase, traditional saying or idiom that expresses this idea.
This poem/proverb is saying that the old friends are gold (more valuable than silver):
The expression the best mirror is an old friend may suggest the idea:
- it means that although a (literal) mirror will show you a picture of your face, an old friend will be honest enough to tell you about your real self, your personality, your good and bad points, etc. That is, an old friend will give you a more reliable "picture" of your real self than a piece of glass or, metaphorically, any other porson would.
Dictionary of proverbs.
You might use the brief phrase friends through thick and thin to describe "best/old friends", while it would never be used for new friends. The phrase not only implies the length of a friendship but also the strength of it. Unfortunately, it is used to actually describe long-term friends, not to differentiate between them and newer friends, so it may not be what you are seeking.