Imperial Topaz. Until the 1950s, topaz was known almost exclusively as a yellow to golden gemstone. But the routine radiation and heat treatment of pale-colored topaz to turn it shades of blue has changed the public’s perception of this gem to that of an affordable blue gemstone. Highly prized “imperial topaz” is an intense golden to reddish-orange color and is found primarily at Ouro Preto, Brazil. The imperial topaz crystal and gem pictured here weigh 875.4 and 93.6 carats respectively.