A method and an apparatus for
laser marking indicia in the volume of gemstones such as diamonds, the indicia being made up of a plurality of microscopic dot-shaped marks whose build-up can be initiated by exposing naturally-occurring internal defects or impurities in the volume of a
gemstone to a tightly focused
train of
laser pulses.
Authentication data is encoded in the
gemstone from the relative spatial arrangement of the dot-shaped marks that form the indicium. Taking
advantage of the presence of otherwise invisible defects in the
gemstone allows for inscribing indicia with
laser pulses carrying energies substantially lower than the
threshold energy required for inscribing in the volume of a perfect gemstone material. The marking process is then much less susceptible to inflict damages to the surface of the gemstone, and the marking can be performed using a broad variety of
femtosecond laser systems. The dot-shaped marks engraved at a depth below the surface of a gemstone can be made undetectable with the unaided eye or with a
loupe by limiting their individual size to a few micrometres, while devising indicia made up of only a few marks. As a result, the marking does not detract from the appearance and value of the gemstone. The procedure for laser marking accounts for the random
spatial distribution of the defects present in natural gemstones as well as for their strongly localized character. The presence of an indicium can be detected by using a dedicated
optical reader that can be afforded by every jewellery store.