Abstract: In order to test whether a diamond has had a layer a synthetic diamond deposited thereon, the diamond is radiated with ultraviolet radiation so as to form a pattern of beams of refracted and reflected radiation, the pattern of refracted and reflected radiation being observed on a screen behind the diamond.

Abstract: In order to test whether a diamond has had a layer a synthetic diamond deposited thereon, the diamond is radiated with ultraviolet radiation so as to form a pattern of beams of refracted and reflected radiation, the pattern of refracted and reflected radiation being observed on a screen behind the diamond.

Abstract: In order to test whether a diamond has had a layer of synthetic diamond deposited thereon, it is irradiated with high energy ultraviolet radiation to cause emission of luminescence, the luminescence intensity produced by different zones of the diamond being measured and compared. In one embodiment an integrating enclosure (15) is used and the diamond (13) is mounted on a rotatable mount (14).

Abstract: In order to test whether a diamond has had a layer of synthetic diamond material deposited thereon, infrared radiation including radiation of wavelength substantially 7 .mu.m to 25 .mu.m preferably 7 .mu.m to 10 .mu.m emitted or transmitted by the diamond is observed, to detect differences between the compositions of different zones of the diamond.

Abstract: In order to test whether a diamond has had a layer of synthetic diamond deposited thereon, the surface area of the natural part of the stone is measured by measuring the radiant-flux density of radiation substantially of wavelength 230 nm to 320 nm in an integrating sphere containing the diamond. This is compared to the total surface area of the diamond.

Abstract: In order to examine a gemstone, the stone is irradiated and observed though a filter (5) at a first wavelength which is characteristic of a first class of gemstones. The filter can be rocked from a setting normal to the optical axis to transmit radiation of at least one reference wavelength. The intensities of radiation transmitted at first and reference wavelengths are observed and compared, to classify the gemstone as belonging to the first class or not. In order to detect whether the gemstone is a diamond or diamond simulant, the first wavelength comprises raman emission characteristic of diamond. In order to detect whether the gemstone is definitely natural or not definitely natural, the gemstone is classified as definitely natural if there is a maximum of absorption of radiation at 415.5 nm.

Abstract: In order to test whether a diamond 2 comprises synthetic diamond material, a plurality of different zones of the diamond 2 are irradiated with radiation substantially of wavelength substantially 230 nm to 320 nm, an image or reading of the radiation transmitted by each zone of the diamond 2 being produced.