At room temperature, diamond is the material with the highest known temperature conductivity, which can reach up to 2100 W/(m K) depending on crystal purity [SEN91]. Furthermore, diamond exhibits a low electric conductivity, which can increase significantly with Boron assembled into the diamond crystal structure. In oxygen atmosphere, diamond has a low friction coefficient of p = 0.05-0.1 with most materials, including diamond itself [MARI04].
2.3.1.1 Electric and Magnetic Properties
Electric and magnetic conductivity of diamond depends on the inclusions [YIN00]. In particular, catalyst inclusions in synthetic diamonds change the electric and magnetic behavior, which can negatively affect electroplating processes or enable to align the abrasive grits on grinding belts or wheels.
Diamond forms only at high pressures and temperatures. Its natural genesis is assumed to happen in depths from 100 to 300 km beneath earth’s surface at temperatures above 1200 °C and pressures above 50,000 bar [EPPL94]. It can be distinguished between primary deposits where diamond is located within parent rock and secondary sources where diamond is accumulated in rivers, in the sea, or in dunes by erosion [EPPL94]. However, the source of diamond has a main influence on the chemical and physical properties. Bigger natural diamond grits are mechanically broken down to smaller grits [FIEL81]. Grit suppliers and tool manufacturers sort natural diamonds and treat their surface to work best in different bonding systems and machining applications. Natural diamonds, especially high-quality diamonds for jewelry are closely related to social sustainability in two ways: Mining practices are to be reviewed and diamonds might be used to feed conflicts.
Natural polycrystalline diamonds are known as carbonado, ballas, boart, framesite, stewartite, etc. They occur in various different structures. Since the synthesis of fragile diamond was invented, the importance of fragile natural diamond for manufacturing technology has decreased.
Jewel industry uses the “4 C’s” to appraise diamond grits, i. e. the criteria carat (weight), clarity (purity), color, and cut. In fine machining, however, grit morphology, purity, and size are most important. In natural diamonds, inclusions of minerals can be found, which were present at diamond genesis. Possible contaminations are SiO2, MgO, FeO, Fe2O3, Al2O3, TiO2, graphite, etc. [MARI04]. Cracks in diamonds can also be of natural origin, e. g. from geological mechanical load [LENZ86].