Sapphire

The Logan Sapphire is displayed in the Smithsonian Museum of Natural History. At 423 carats, this blue sapphire is the largest one on public display. The name sapphire comes from the Greek word "sappheiros" for blue.

Sapphire is a type of the mineral corundum. In fact, any gem-quality corundum is called sapphire unless it is deep red, and then it is called ruby. Corundum is Al₂O₃, and it is clear and transparent if there are no impurities. It has a refractive index of 1.76-1.78 and a hardness of 9. Its crystal symmetry is trigonal. A trace of chromium creates pink, iron creates green and yellow, chromium plus iron creates orange, and titanium plus iron creates blue. This blue sapphire color is difficult to render by a monitor, so the color has to be seen to be believed.