A cleavage is a break parallel to the weakest direction in the gem structure. This weak place is called the cleavage plane of the diamond. A cleavage sometimes has a step like appearance if it's not a clean break. A cleavage flaw can have a large impact on the value of a diamond as it can affect the durability of the stone as well as impact it's appearance. Gem cutters take advantage of the cleavage plane in a rough diamond when they aim to achieve a clean cut along the edge of a gemstone.