Basic Properties of Diamond

by Hanjiang Wang, Diamond Blade Select

Diamond can be either natural or synthetic, and has the following basic properties:

		Description
Mechanical Properties	Hardness	Diamond is almost the hardest material on the earth. On the Mohs scale of mineral hardness, its hardness is 10 — the hardest. Its microhardness is about 100000 MPa. Its Rockwell hardness is about 1.56 times of CBN’s, 3.7 times of tungsten carbide’s, 4.4 times of corundum’s and 8.5 times of quartz’. Diamond’s hardness is different on its different orientations (crystal planes). Because of its very high hardness, diamond can be used to make various diamond tools such as diamond saw blades, diamond core drill bits, diamond cup wheels and diamond polishing pads, to cut, grind or polish hard or abrasive materials, e.g., stone, concrete, asphalt, ceramics, bricks and carbide alloy.
	Brittleness	Though diamond is very hard, it is relatively brittle. It splits along its crystal cleavage planes when receiving a certain impact. The brittleness of diamond is also related to its crystal completeness. Crystal defects can generate internal stress and even lead to natural split. While complete crystal has a fairly high toughness and its split pressure can reach 30 – 100 MPa. So “impact toughness” is one of the main indexes to measure the quality of industrial diamonds.
	Density	The density of diamond is normally 3.47 – 3.56 g/cm³. Pure and complete-crystal diamond’s density is 3.52 g/cm³.
	Cleavage	The cleavage of diamond happens most likely along the <111> plane (a triangle that consists of the three diagonals of the three adjacent planes of a cubic diamond lattice).
	Fracture	The fracture surface of diamond can have a complex structure, and is often conchoidal or irregular-shaped.
Optical Properties	Color	Pure diamond has no color, but it is fairly rare. The color of diamond is often yellow, green, brown, rose, blue, grey and black. This is related with the impurities it contains and its crystal defects.
	Luster	Diamond often has the so-called “diamond gloss”, which means quite strong reflected light, shining but not metallic. A few diamonds have oil sheen, glass gloss or even no luster, which is usually caused by long-term chemical corrosion, radiation effects, invasive alien substances or other materials coated on their surface.
	Transparency	Pure diamond is transparent, and impure diamond can be translucent or even opaque.
	Refractive index	Pure diamond’s refractive index is 2.40 – 2.48, which is the highest in transparent minerals. The higher refractive index, the higher reflectivity. Diamonds that are specially designed and processed can reflect almost all the light that comes to their sides and interior.
	Dispersion	Diamond’s dispersion coefficient is 0.063, which is also the biggest in transparent minerals. The bigger dispersion coefficient, the better spectroscopy result. When a white light comes into a well-cut diamond, the light can be split into different colors of light because of the diamond’s dispersion effect, and the diamond looks dazzling.
	Luminescence	Diamond can shine distinct green, sky-blue or blue fluorescence under cathode ray, medium-brightness or weak sky-blue fluorescence under X-ray, distinct or medium-brightness sky-blue, purple or yellow-green fluorescence under ultraviolet, and light green-blue phosphorescence in darkroom after exposure under sunlight.
Thermal Properties	Thermal conductivity	Diamond is a good heat conductor. Different types of diamonds have different thermal conductivity. Some diamonds have super thermal conductivity.
	Thermal expansion	Diamond’s thermal expansion coefficient is very small at low temperature, but increases rapidly with the rising of temperature.
	Heat resistance	Diamond’s ignition point is 720 – 800 °C in oxygen and 850 – 1000 °C in air. The flame is blue when diamond is burning.
Electromagnetic Properties	Magnetism	Pure diamond is not magnetic. But as containing impurities of catalyst alloy, e.g., Ni, Co, Fe, Mn, Cr, synthetic diamond is magnetic. The more catalyst impurities, the more magnetic the synthetic diamond is. Low magnetism indicates the diamond’s good crystal and its good quality.
	Electrical conductivity	Normally, diamond is a poor electrical conductor. Its electrical conductivity increases when temperature rises. Some diamonds are good semiconductor.
	Photoconductivity	When diamond is irradiated by ultraviolet of 2.1 x 10^-4 – 3.0 x 10^-4 mm wavelength, there are photocurrents generated in the diamond.
	Triboelectricity	When diamond rubs on the surface of glass, hard rubber or plexiglass, positive charge can be produced on the diamond.
Surface Properties	Lipophilicity & Hydrophobicity	Diamond cannot be wet by water, but can be easily wet and stuck by oil. This property can be utilized to extract diamonds using oil when making synthetic diamonds.
Chemical Properties	Chemical stability	Diamond has very good chemical stability. It does not react with acid and alkali under room temperature. Its surface can only be oxidized a little by just a few oxidants under high temperature (below 1000 °C). So acid and alkali can be used to refine synthetic diamonds.

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