In an octahedral diamond crystal the most obvious grains are the edges of the triangular faces. Imaginary lines exist all the way to the center of the crystal. The outside arrows represent a cutter’s polishing directions perpendicular to the grains.
When grain is mentioned in relation to diamond, it is usually cleavage grain that a dealer is referring to. This is a very narrow definition, but it is at least the beginning of what grain in a diamond is all about. As you can see in the illustrations herewith, the octahedral crystal that characterizes the diamond structure is delineated by eight triangular faces that make up two pyramid shapes joined at their bases. The lines making up the triangles represent single grains that travel all the way through a diamond crystal in their pre-established paths. The diamond’s molecular structure has some similarity to the grain in wood. If you split a piece of wood, the plane of division must travel parallel to the grain. The same rule applies to diamond, except that there are twelve individual, precisely-oriented grains. A cutter (or an unsuspecting victim) must split, cleave or break a diamond along one chosen grain. In the polishing process (which will be discussed in more detail later), the cutter must polish (grind) the stone perpendicular to the dominant grain.