Diamond is the hardest material on Earth(1). Nevertheless, polishing diamond is possible with a process that has remained unaltered for centuries and is still used for jewellery and coatings: the diamond is pressed against a rotating disc with embedded diamond grit(2). When polishing polycrystalline diamond, surface topographies become non-uniform because wear rates depend on crystal orientations(3). This anisotropy is not fully understood(4) and impedes diamond's widespread use in applications that require planar polycrystalline films, ranging from cutting tools(5) to confinement fusion(6). Here, we use molecular dynamics to show that polished diamond undergoes an sp(3)-sp(2) order-disorder transition resulting in an amorphous adlayer with a growth rate that strongly depends on surface orientation and sliding direction, in excellent correlation with experimental wear rates(7). This anisotropy originates in mechanically steered dissociation of individual crystal bonds(8). Similarly to other planarization processes(9), the diamond surface is chemically activated by mechanical means. Final removal of the amorphous interlayer proceeds either mechanically or through etching by ambient oxygen(10).