Diamond rough is formed miles beneath the surface of the Earth over many years. With extreme heat and pressure, carbon atoms fuse together and then quickly cool, forming crystals. These crystals are known as rough diamonds, or raw diamonds. The terms are really interchangeable, and you may see one or both being used to describe diamond crystal.
In this raw form, rough diamonds usually look like lumps of pale, colored glass. Although colorless or near colorless diamonds are a standard for cut, faceted, and polished diamonds, rough diamonds often carry some hints of color. This color usually happens when trace elements of other minerals are present in small quantities at the time of formation.
In rough and raw diamonds, this feature is often one of the most appealing. The tiny surface inclusions, streaks of color, or cloudy patches immediately recall the life of the diamond growing process and spur a direct connection to nature.
Diamonds are known for their strength and hardness, and rough diamonds are no different. This is the clearest way to identify a rough diamond. Most often, rough diamond is tested against corundum, the mineral species which makes up ruby and sapphire. Only diamond is harder, so if the stone is able to scratch corundum, it must certainly be a diamond.