Back when we first discovered diamonds, there was no known way to cut one. What people did was place the gem into jewelry in the same form as it was when it came out of the ground. The Romans even believed that cutting a diamond would deprive it of its mystical powers, which included protection to the wearer.
Over time, people came to realize that one diamond could cut or polish another, and sometime after that, the basic cut styles began to emerge. The rose cut was one of the first popular cuts. It had facets, which come together at a high point, with the underside being flat.
In the 17th and 18th Centuries, Africa was discovered to be a hotspot for diamonds, and with the sheer number of rough stones coming in, people became interested in making new shapes which reflected light in a better way. The cuts resulting from this formed the precursors for the modern cuts we find today.