A process of forming a non-optically detectable authentication marking (110), includes the step of: applying a marking at a surface of a diamond (200) using a focused ion beam (FIB) writing process so as to provide a non-optically detectable authentication marking (110) which is formed by alteration in the optical characteristics of a portion of the diamond material at the outer surface of the diamond to form a marked portion; wherein the marking is optically invisible, and wherein the marking is viewable by an imaging method which provides an observable contrast between the portion of the diamond having altered optical characteristics and the non-marked portion of the diamond. The marking process can assist in the prevention of the counterfeiting of precious articles, and be of assistance in the incident of theft. A marking, a diamond, a process of viewing a marking on a diamond are further disclosed.