What natural resource made Ghana a prosperous kingdom?

Natural resource which made Ghana a prosperous kingdom is gold. This country had abundance of gold, since it literally sat on the gold mine, and its rulers used it for many years of successful trading. Therefore, the kingdom became powerful and prosperous. In some periods even dogs in Ghana had golden collars, and Ghana's kings were among the richest men in the world. They traded gold for salt (mostly with Arabs) which was very important for people's health. Today, Ghana is the second largest gold producer in Africa.