<Tr> <Th> Ultraviolet fluorescence </Th> <Td> Red under longwave </Td> </Tr> <Tr> <Th> References </Th> <Td> </Td> </Tr> <P> A ruby is a pink to blood - red colored gemstone, a variety of the mineral corundum (aluminium oxide). Other varieties of gem - quality corundum are called sapphires . Ruby is one of the traditional cardinal gems, together with amethyst, sapphire, emerald, and diamond . The word ruby comes from ruber, Latin for red . The color of a ruby is due to the element chromium . </P> <P> The quality of a ruby is determined by its color, cut, and clarity, which, along with carat weight, affect its value . The brightest and most valuable shade of red called blood - red or pigeon blood, commands a large premium over other rubies of similar quality . After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle - like rutile inclusions may indicate that the stone has been treated . Ruby is the traditional birthstone for July and is usually more pink than garnet, although some rhodolite garnets have a similar pinkish hue to most rubies . The world's most valuable ruby is the Sunrise Ruby . </P>

Red colour of ruby crystal is due to