What a great question. Normally the word 'rich' refers to a person's wealth, so you'll usually encounter it in any discussion about money. However, the phrase 'rich in' is commonly used when describing a nation's wealth. For example, Saudi Arabia is rich in oil, or India is rich in coal. 'Rich with' is usually used more metaphorically to describe the fact that the subject has a lot of something. It's less literal. For example, a person may be rich with knowledge, or the air can be rich with a scent (smell). A field can be rich with flowers, or a good curry can be rich with a particular ingredient. I hope that helps, and thanks again for your great question.