The heat conductivity of diamond exceeds that of most its imitations. Only one imitation, moissanite, has almost the same heat conductivity as diamond. This property of diamond makes possible heat conductivity-based diagnostics.

   The heat conductivity-based diagnostics allows one to distinguish diamond and moissanite from all the other imitations. When examining a stone whose carat weight is less than 0.03, the accuracy of this method is not enough to consider the measured heat conductivity value as a direct diagnostic feature. In this case, other diagnostic methods should be applied. To measure the heat conductivity of the sample under study, Duotesters or Diamond-detectors are used.

   The heat conductivity should be measured at several crown and pavilion facets of the sample. All the measured values should be close. If the heat conductivity test reveals that some portion of the sample is not diamond, this means that this is a composite stone.

---

   Diamond, as well as any its imitation, is characterized by a certain value of reflectivity, which is connected with the refractive index of the material. Reflectivity measurements provided by special instruments called Duotesters are widely used in diagnostics of diamonds.

   The reflectivity of diamond is close to that of moissanite. Therefore, in most cases, the reflectivity test fails to distinguish between diamond and moissanite. The reflectivity of a stone is its indirect diagnostic feature.

   It is possible to conclude whether the studied sample is a diamond or not by comparing the measured value of its reflectivity with the reflectivities of various materials, which can be found in a special table attached to the Duotester.

---

   Diamond is a singly refractive material. The diagnostic method based on this property allows one to unambiguously distinguish a diamond from any doubly refractive imitation but fails to discriminate between a diamond and a singly refractive imitation. The optic character of a stone is its indirect diagnostic feature. To test the optic character of the studied sample, polariscopes are used.

   The double refraction of the studied sample is often best observable when laterally viewing the stone (from the side of its girdle). In this case, a composite stone may be singly refractive within one portion and doubly refractive within another.

   The crystals of cubic syngony (including diamond) and glasses are singly refractive. However, they often show anomalous birefringence when viewed between crossed polarization filters. This birefringence is due to stresses in the crystal structure of the material. In such a case, instead of clear successive brightening and darkening of the studied sample, alternating dark and bright spots or stripes, or dark grating are observed. Synthetic spinel always shows anomalous effects, such as wavy darkening. Due to stresses that often arise in glasses, a dark cross is often observed in a glass.

   Being a doubly refractive mineral, moissanite brightens and darkens four times when rotated between two crossed polarization filters. This feature is enough to distinguish between moissanite and diamond.

   If the polariscope test confirms the double refraction of the studied sample, this is enough to conclude that the sample is an imitation. Other results of the test do not allow one to make a final conclusion.

---

   Diamond, as well as any its imitation, is characterized by a certain value of specific gravity (density), which is connected with the structure of the material. Measuring the specific gravity is used in diagnostics of diamonds. The specific gravity is a direct diagnostic feature. To measure the specific gravity of a stone, a balance with hydrostatic weighing option is used.

   It is possible to conclude whether the studied sample is a diamond or not by comparing the measured value of the specific gravity with data contained in Table 2.

   If the hydrostatic weighing option is unavailable, another density-based diagnostic method can be used. It implies weighing the sample and comparing the measured mass with the value determined by a special formula used to calculate the mass of diamonds. If these two values are equal, the sample is a diamond. If they differ by more than 10%, the sample is an imitation.