A process of forming a non-optically detectable authentication marking (210,320, 410,535) in a diamond (200,300). Authentication marking (210,320,410,535) is formed adjacent the outer surface of an article formed from a diamond material having intrinsic optical centers. Method includes the step of applying an image of predesigned authentication marking(210,320,410,535) to a region (201,310,530) of a diamond (200,300) at or adjacent the surface of the diamond (200,300) by way of a direct laser writing; wherein the fluorescence background of the diamond material from intrinsic optical center is suppressed by authentication marking(210,320, 410, 535) under fluorescent imaging, such that the non-optically detectable identifiable authentication marking (210,320,410,535) is viewable against the fluorescence background at the region (201,310,530) of the diamond (200,300) where the authentication marking (210,320,410,535) is applied.