Diamond is transparent to opaque, optically isotropic, 3D-crystalline carbon. It is the hardest naturally occurring material known, owing to its strong covalent bonding, yet its toughness is only fair to good due to important structural weaknesses. The precise tensile strength of diamond is unknown. However, strength up to 60 GPa has been observed, and its theoretical intrinsic strength has been calculated as 90 to 225 GPa, depending on the crystal orientation.[1] Diamond has a high refractive index (2.417) and moderate dispersion (0.044), properties which are considered carefully during diamond cutting and which (together with their hardness) give cut diamonds their brilliance and fire. Scientists classify diamonds into two main types and several subtypes, depending on the nature of crystallographic defects present. Trace impurities substitutionally replacing carbon atoms in a diamond's crystal lattice, and in some cases structural defects, are responsible for the wide range of colors seen in diamond. Most diamonds are electrical insulators but extremely efficient thermal conductors. The specific gravity of single-crystal diamond (3.52) is fairly constant. Contrary to a common misconception, diamond is not the most stable form of solid carbon; graphite has that distinction.
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