"Jewel grade" as I understand the term, refers to a gemstone which can be used as is, or with minimal cutting and polishing. The problem with diamonds is that, being the hardest natural substance, there is no way to polish them. Softer gems such as quartz can be polished by tumbing in water and some polishing agent, such as jewler's rouge or diamond dust. However, diamonds are not indestructable as the ancients believed. If one knows their basic crystal structure--in the case of diamonds a slightly convex octohedron--one can cut a diamond by a sharp blow along their planes of cleavage. Strike the blow in the wrong direction and the diamond will shatter. European jewel smiths did not discover this until about 1380. In 1475 the Duke of Burgundy commissioned Louis de Berquem to cut three diamonds, one of the being the "Sancy" diamond. de Berquem developed a 32-facet cut, the ancestor of the modern "brilliant" cut. So impressed was the duke that he included the diamonds in his crown jewels, thus creating a new fashion rage for the courts of Europe. source: Henri-Jean Schnubel, "Gems & Jewels"