Diamond are the hardest natural substance known. Diamonds earliest known history dates back to a Sanskrit manuscript called the Arthasastra (The Lesson of Profit) by Kautiliya, a minister of Chandragupta of the Mauryan dynasty in Northern India. The manuscript dates back to 320-296 b.c.e (before common era).
Diamonds are made of carbon. This carbon sinks deep into the Earth by tektonic plate movement where the intense heat and pressure of the Earth transforms the carbon into diamonds. Once the diamond is formed, it is stuck there. It may take millions of years before the right conditions bring the diamond to the surface of the Earth. There is only one thing that commonly occurs that can bring diamonds up from deep inside the Earth. That is volcanoes.
Diamonds are found near areas of volcanic activity. Because of the depths required to form diamonds, it is impossible for us to dig to those depths. Fortunately, volcanoes periodically bring material from deep within the Earth such as garnet, zircon and diamonds.