Diamond differs from all the imitations in hardness, which is extremely high for diamond. That is why a cut diamond differs from an imitation in the quality of its finishing (especially polishing). Diamonds are also characterized by the presence of distinctive internal defects and inclusions. The finishing quality, internal defects, and inclusions may serve as direct or indirect diagnostic features.

   To study the finishing quality, internal defects, and inclusions, a gemological loupe (a 10x triplet) or a binocular gemological microscope is used.

   When viewing the studied sample under magnification, pay attention to the following indirect diagnostic features:

- Imitations often have abraded culet and edges, polishing traces, scratches and marks on facets, chips, non-flat facets, and curved edges. All these defects are not typical of diamonds.

Chip on the surface of glass imitation.

- Diamonds have characteristic inclusions (for example, pyrope, olivine, chrome diopside, some sulfides, diamond) that can be identified by appearance, size, shape, color, transparency, and relief.

Two cracks under table in a polished diamond

Large crack in a polished diamond

Red garnet (pyrope) included in a polished diamond

Red garnet (pyrope) included in a polished diamond

Inclusion of spinel crystal in a polished diamond

Group of white pinpoints in a polished diamond

- Diamonds have characteristic features of internal structure, such as shear planes, twinning planes (twinning wisps), structure inhomogeneities, and some others.

Dark twining line in a polished diamond

- When viewing a birefringent sample through its table, both inclusions and pavilion edges "double". Diamond is a singly refractive mineral (that is, shows no birefringence). Therefore, if testing a stone reveals its birefringence, it is possible to conclude that this is an imitation.
- Gluing traces, gas bubbles, or striaes in a gluing plane are features of a composite stone.

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   The "dot method" is based on total internal reflection of light, which takes place in round brilliant cut diamonds having accurate proportions. This method is applicable to transparent samples. It reveals all imitations with refractive indices lower than that of diamond and provides a gemologist with an indirect diagnostic feature.

   The sample under study is put on a dot drawn with a dark ink on a white paper sheet. The table of the stone is bottom. For an imitation having a low refractive index, the dot can be seen through pavilion facets. For a diamond or an imitation having a high refractive index (moissanite, strontium titanate, rutile), there is no way to see the dot though pavilion facets. A disadvantage of this method is the fact that some cut styles and certain pavilion angles prevent the total internal reflection.

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   The most common color of diamond fluorescence is blue. However, the fluorescence can be also yellow, green, pink, or other. The color and intensity of the fluorescence are indirect diagnostic features. To excite the fluorescence, an ultraviolet lamp is used, the radiation of which has a wavelength of 365 nm.

Blue long- wave U.V. fluorescence in a polished diamond

   The observation of the fluorescence of the studied sample should be performed in a darkened room or after screening foreign illumination sources.

   The color and intensity of the fluorescence as well as its distribution in the bulk of the sample are determined.

   It is possible to conclude whether the studied sample is a diamond or not by comparing the results of the test with the UV fluorescence data contained in Table 2. If the observed fluorescence color is blue or yellow, the sample is possibly a diamond. If the fluorescence color is different (for example, red) and its intensity is high, the sample is probably an imitation. If there is a considerable difference between the color and/or intensity of the fluorescence from crown facets and pavilion ones, the sample is most likely a composite stone.

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   Most natural imitations of diamond have relatively low (less than 1.81) refractive indices. This fact is used in diagnostics of diamonds. Refractive index measurement allow one to unambiguously distinguish between a diamond and an imitation with a refractive index lower than 1.81 but fails to reveal an imitation with a refractive index higher than 1.81. The refractive index of a stone is an indirect diagnostic feature. To measure the refractive index, a refractometer is used.

   If the measured value of the refractive index of the studied sample is lower than 1.81, it can be concluded that the sample is an imitation. Otherwise, the refractive index test fails to provide a final conclusion.

   To increase the accuracy of the diagnostics, it is necessary to measure the refractive indices both at top and bottom facets of the stone. If the refractive indices considerably differ, the sample is most likely a composite stone.

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Method	Instrument	Action	Discrimination	Notes
Heat conductivity measurement	Duotester or Diamond-detector	As a result of the test, a pointer moves across a scale (or a signal lamp lights up)	All imitations except moissanite; composite stones	Applicable for express-diagnostics of mounted and non-mounted stones
Reflectivity measurement	Duotester	A display shows the reflectivity value	All imitations except moissanite	Applicable for express-diagnostics of mounted and non-mounted stones
Specific gravity measurement	Hydrostatic balance	Weighing in a liquid and in the air (or immersion into a liquid of a known density)	Imitations whose density differs from that of diamond	Applicable to non-mounted stones only
Visual observation	10Õ loupe or microscope	Viewing the stone under magnification	Imitations with distinctive features of the surface and interior; composite stones	Applicable for diagnostics performed by a qualified expert
"Dot method"	White paper sheet with a black dot drawn	The stone is put on the dot (the table is down) and viewed from the pavilion side	Imitations whose refractive index is less than that of diamond	Applicable only to round brilliant cut diamonds with accurate proportions
Fluorescence test	UV-lamp	Fluorescence color and intensity are determined	Imitations from fluorescent diamonds; composite stones	Indirect diagnostic feature
Optic character test	Polariscope	Rotation of the stone sandwiched between two crossed polarizers	All doubly refractive imitations	Applicable for laboratory diagnostics of non-mounted and open mounted stones
Refractive index measurement	Refractometer	The refractive index is determined at a smooth polished surface and displayed on a scale	Imitations whose refractive index is less than 1.81; composite stones	Applicable for stones having a flat facet

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Material and its chemical formula	Optic character	Refractive index	Hardness	Dispersion	UV fluorescence	Specific gravity	Additional features
Diamond Ñ	Singly refractive	(2,42)	10	0,044	Blue, green, yellow (the intensity varies), or absent	3,52	High heat conductivity, anomalous birefringence, mineral inclusions
Phianite ZrO2	Singly refractive	(2,18)	8,5	0,060	Weak yellow-orange or absent	5,95	Gas bubbles (seldom)
Zircon ZrSiO4	Doubly refractive	1,78-2,01	6-7,5	0,038	Moderate yellowish, orange, whitish	3,95-4,80	Mineral inclusions, strong birefringence (edge doubling)
Strontium titanate (fabulite) SrTiO3	Singly refractive	(2,41)	6	0,109	Absent	5,13	Very strong dispersion, gas bubbles, sharp traces of polishing
Yttrium-aluminum garnet (YAG) Y3Al5O12	Singly refractive	(1,83)	8,.5	0,028	Orange	4,55	Gas bubbles
Gadolinium gallium garnet (GGG) Gd3Ga5O12	Singly refractive	(2,03)	6,5	0,022	Weak orange (S.W.)	7,05	Gas bubbles
Lithium niobate (linobate) LiNbO3	Doubly refractive	(2,25)	5,5	0,075	Absent	4,65	Perfect cleavage, gas bubbles, strong birefringence (edge doubling)
Synthetic rutile TiO2	Doubly refractive	(2,62-2,90)	6-6,5	0,190	Absent	4,26	Gas bubbles, strong birefringence
Yttrium aluminate YÀlO3	Singly refractive	(1,97)	8,5	0,033	No data	5,35	Gas bubbles, no fire
Glass	Singly refractive	1,44-1,90	5-6	0,009-0,098	Various	2,3-4,5	Abraded edges gas bubbles, anomalous birefringence
Quartz SiO2	Doubly refractive	1,544-1,553	7	0,008	Absent	2,66	Gas-liquid inclusions
Natural and synthetic sapphire (leucosapphire) Al2O3	Doubly refractive	1,76-1,77	9	0,011	Moderate blue (S.W.) or absent	4,00	Very weak twinning, gas bubbles, inclusions
Synthetic spinel MgAl2O4	Singly refractive	1,73	8	0,010	Various (from inert to strong)	3,64	Gas bubbles, polishing traces, anomalous birefringence ("tatami" pattern)
Synthetic scheelite CaWO4	Doubly refractive	(1,92)	4,5-5	0,015	Pink, blue (S.W.)	6,06-6,30	Curved growth lines, gas bubbles
Topaz Al2SiO4 (F,OH)	Doubly refractive	1,61-1,62	8	0,014	From weak to inert	3,53 - 3,56	Mineral and gas-liquid inclusions
Sphen (titanite) CaTi[SiO4]O	Doubly refractive	(1,84-2,11)	5-6	0,051	Inert	3,29-3,56	Strong birefringence (edge doubling), fragility
Zincite ZnO	Doubly refractive	(2,02)	4,5	0,060	Green	5,55	Fragile
Moissanite SiC	Doubly refractive	(2,69)	9,5	0,090	Inert or weak yellowish	3,10-3,22	Strong birefringence (edge doubling)