Gold was the first metal known. It was first discovered in its natural state, in streams all over the world. Some qualities of gold make it very interesting and easy to work with. It is a natural beauty. It is lustrous and so has a very good shine. It is malleable and hence can be easily beaten into sheets. It is ductile and thus can be easily drawn into wires. It is a very inert material and does not react easily and its resistance to tarnish make it enjoyable to work and play with. Eventually, gold became a part of every human culture.
Another reason why gold was so widely used was that it is the easiest of the metals to work. It occurs in a virtually pure and workable state, whereas most other metals tend to be found in ore-bodies that pose some difficulty in smelting. The first use of gold was limited to ornaments only.
Properly measured gold became money. Gold's beauty, scarcity, unique density, and the ease by which it could be melted, formed, and measured made it a natural trading medium. Gold gave rise to the concept of money itself. Gold replaced barter system and hence became currency.
Whereas, when it comes to diamonds, though it is very shiny and attractive, but what you see is not so simple. Diamonds are created through a complex process. It is a tetrahedral shaped isomer of carbon which takes billions of years to come to existence. Also, it is very difficult to shape diamonds. It was in 1375 that people discovered the point cut technique to cut and shape a diamond. Also, there’s no sensible way to measure it.