Introduction

Diamond is a gemstone, which is a crystal of carbon. Diamond crystals are also grown in the laboratory. Natural diamonds come in a variety of colors, including white, yellow, pink, and blue. Yellow, pink, and blue diamonds are rare and hence expensive. Colors of diamonds are caused by optical absorptions due to impurities (nitrogen, hydrogen, boron) and defects. Nitrogen is responsible for the yellow color of diamonds. Boron is responsible for the blue color of diamonds. Diamond is a large-gap semiconductor with an indirect gap of 5.48 eV (226 nm). The density of diamond is 3.515 g/cm³. Diamond is harder and conducts heat better than any other known material. The relative hardness of diamond is 10 Mohs. The thermal conductivity of diamond at 300 K is 900 W/m K compared to that of 386 W/m K for copper.

Both ruby and sapphire are corundum (α-aluminum oxide) crystals. Ruby is red, which is due to chromium. Sapphire is typically blue. Blue sapphires are due to the absorptions of iron and titanium pairs. The physical properties of ruby and sapphire are the same. Sapphire belongs to the trigonal crystal system. The lattice constants are a = 4.785 and c = 12.991 A. The density of sapphire is 3.98 g/cm³. The melting point of sapphire is 2303 K. The relative hardness of sapphire is 9 Mohs, which is the second-hardest material. The optical transmission of synthetic color-free sapphire is 0.17–5.5 μm. The specific heat of sapphire is 761/kg K at 291 K. Values of the thermal conductivity of sapphire at 300 K are 23.1 W/m.K and 25.2 W/m.K.