Diamonds were formed over 3 billion years ago deep within the Earth's crust under conditions of intense heat and pressure that cause carbon atoms to crystallise forming diamonds. ... This expansion causes the magma to erupt, forcing it to the Earth's surface and taking along with it diamond bearing rocks.
Do diamonds come from coal?
Over the years it has been said that diamonds formed from the metamorphism of coal. According to Geology.com, we now know this is untrue. “Coal has rarely played a role in the formation of diamonds. ... The diamonds form from pure carbon in the mantle under extreme heat and pressure.6 nov 2014