Diagnostics of cut diamonds implies successive diagnostic tests and comparison between the properties of the sample under study with those of diamond and its imitations (Table 2). The diagnostics results in concluding whether the sample is a diamond or an imitation. All the diagnostic methods are divided into primary and secondary ones. The primary diagnostic methods are most common and provide quick diagnostics. To make the diagnostics unambiguous, it is necessary to successively run several tests based on measuring the following physical properties of the studied sample: heat conductivity, refractivity, optic character (singly refractive or doubly refractive), and specific gravity.

   The secondary diagnostic methods are used in those cases when the results of the primary diagnostic methods do not allow a gemologist to make an unambiguous conclusion or when it is desirable to additionally check these results.

   A cut gemstone to identify should have a clean surface. To avoid soiling the stone, it is recommended to handle it with tweezers while running the diagnostic tests.

   Diamond has the following distinctive features:

• High hardness (the hardness of diamond is 10, which is the maximum value for minerals). Diamond shows hardness anisotropy, which means that the hardness depends on the direction of an external action. Due to the high hardness, the facets of a diamond remain polished for years, and its edges remain sharp.
• Singly refractive optic character. Being a crystal of cubic syngony, diamond is singly refractive. However, anomalous birefringence can be often observed in it.
• Perfect cleavage along (111) direction.
• Characteristic set of mineral inclusions.
• Oil wettability. In comparison with imitations, diamond is highly wettable by oil.
• High refractive index (2.42) but relatively small specific gravity (3.52). Most imitations are heavier.
• High heat conductivity (comparable with that of metals). Diamond-detectors are based on this feature.
• Transparency for X-rays. Diamond slightly emits blue light when exposed to X-rays.
• Luminescence excited by ultraviolet radiation. The intensity and color of the luminescence vary.
• High reflectivity (0.17 and higher).
• Strong dispersion of light (0.044).
• Total internal reflection of light, ensured by round brilliant cut.

	Primary diagnostic methods
	Secondary diagnostic methods