London Blue Topaz vs Swiss Blue Topaz: Which Gem Should You Pick? The gemstone known as London Blue Topaz captivates the eye with its deep, rich blue hue that is reminiscent of the Thames River, which flows through the center of London. Due to its remarkable quality and rarity, this gemstone is prized by jewelry aficionados and collectors alike. One member of the Topaz family, a mineral composed of fluorine and aluminium, is the London Blue Topaz.