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Physical Properties of Diamond

Crystalline Form of Diamond

Diamonds are often found in perfect crystals which belong to the regular system, being modifications of the tetrahedron or octahedron, such as the rhombic dodekahedron, hexakistetrahedron, and hexakisoctahedron. South African diamonds are generally octahedral, Brazilian cubical. Sometimes interpenetration or twinning of crystals is observed. The faces of the crystals are often rounded; the fracture may be conchoidal. Cleavage also takes place along directions parallel to the crystal faces. It is the work of the lapidary to cut and polish the diamond so as to develop the brilliancy of the crystal faces. This is done by pressing the surface of the stone against a revolving metal wheel on which is diamond dust mixed with oil.

Hardness of Diamond

The diamond is the hardest known substance; it is "adamant." Its hardness is reckoned 10 on the mineralogist's scale (Moh's scale, largely used by mineralogists, is as follows: (1) Talc, (2) Gypsum, (3) Calcspar, (4) Fluorspar, (5) Apatite, (6) Orthoclase, (7) Quartz, (8) Topaz, (9) Corundum, (10) Diamond.). Nevertheless metallic tantalum and its alloy compete with diamond in hardness, and boride and silicide of carbon abrade the diamond. Different diamonds differ in hardness; e.g. those from Borneo are said to be harder than those from Australia, and Australian diamonds are harder than African. Different faces of the same diamond also differ in hardness. Carbonado is harder than colourless diamond.


The density of the diamond approximates at atmospheric temperature to 3.5, the highest observed value being 3.56 and the average about 3.52. Carbonado, which contains about 2 per cent, of impurity, has a density of 3 to 3.5.

Optical Properties of Diamond

The brilliant lustre and play of colours exhibited by the diamond are due to its high refractive and dispersive power. The refractive index of diamond for sodium light is 2.417. The brilliance of the gem is due to total reflection of light within it on account of its high refractive power; for light incident at the surface from within at an angle greater than 24½° is reflected back into the stone. The corresponding angle for glass is 40½°. The dispersive power of the diamond is shown by comparing its refractive indices for red and blue light, which are 2.402 and 2.460 respectively. Diamonds are sometimes doubly refractive on account of internal strain. From the same cause they may explode spontaneously. Diamond shows absorption bands,3 and those at the blue end of the spectrum are supposed to be due to some rare element such as samarium. Diamond is transparent to Rontgen rays, to which paste imitations are opaque.

Boyle, in 1663, observed that diamond phosphoresces in the dark by friction; it is luminous also after exposure to sunlight, and phosphoresces strongly under the influence of kathode, Rontgen, BecquereL and radium rays.

The radiation spectrum of diamond glowing in a Crookes' tube is continuous, with intensive lines in the green at 537 μμ, and in the blue at 513 μμ and 503 μμ. During this treatment the stone becomes superficially coloured and changed into graphite, and the same effect is produced by the prolonged action of radium. A pale yellow diamond was changed to bluish-green when embedded in radium bromide for eleven weeks.

The coefficient of linear expansion of the diamond is 0.00000118 at 40° C. and 0.00000132 at 50° C., and. becomes continuously less as the temperature falls. The compressibility between 100. and 500 atmospheres is 5×10-7 vol./atm.

The specific heat of diamond varies much with temperature. According to H. F. Weber its value may be calculated from the interpolation formula

γt = 0.0947 + 0.000994t – 0.00000036t2,

whence the following values are obtained:

t° C.50°100°150°200°

At low temperatures the specific heat is much diminished,1 and at the temperature of liquid hydrogen it is only about one-twentieth of its value at atmospheric temperature.

Conductivity for Heat and Electricity

Diamond is a good conductor of heat, and feels colder than glass; it is, however, a bad conductor of electricity, thus differing from graphite.

Action of Heat upon, and Combustion of the Diamond

Diamond is unchanged when heated to whiteness in hydrogen, but swells up and is converted into graphite when placed between the carbon poles of an electric arc. Diamond commences to form carbon dioxide when heated to 720° C. in oxygen; the rate of combustion increases up to 800° C., when a flame appears and the mass becomes incandescent. A minute reddish ash remains, which amounts to from 0.05 to 0.2 per cent, of the original weight of the diamond, and consists of ferric oxide and silica, generally also with lime and magnesia.

Action of Chemical Reagents on the Diamond

The diamond is scarcely attacked by any chemical reagent. It is stable towards chlorine, hydrochloric, hydrofluoric, and sulphuric acids at all temperatures, as well as towards a mixture of nitric acid and potassium chlorate, which dissolves graphite. Fusion with sodium or potassium carbonate, however, converts diamond into carbon monoxide by the reaction

C + CO2 = 2CO.

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