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. It is a durable gemstone that may be worn for a very long time because it is a hard stone, with an 8 on the Mohs hardness scale. This topaz's blue hue is the result of subjecting it to high levels of intensity, which gives it a deep, rich blue tint that is unusual for certain other gemstones.