London Blue Topaz is a form of topaz that has been treated; irradiated and heated. Natural blue topaz does exist in nature but is very rare and expensive, so most blue topaz on the market has been treated in this way. Treated blue topaz comes in different shades; Sky Blue (light), Swiss Blue (mid blue) and London Blue, which is a medium to dark greenish to grayish blue. It's a really attractive colour, quite unique, and it looks fantastic with both white and yellow metals. More expensive than its lighter counterparts, this is due partly because in general, more saturated colours generally are more valuable than lighter ones, but also to the fact that the treatment process is longer. Because the stones are irradiated, they need time to cool off and London Blue topaz has an 18 month cooling off period. All of my blue topaz is treated in the US in line with safety guidelines there so it is all completely safe. London Blue Topaz has become really popular recently, but due to the time taken to produce it, suppliers are unwilling to invest more to keep up with demand against the possibility that it falls from favour and they can't shift the goods. This has caused a sharp rise in price over the past couple of years that currently and unfortunately shows no signs of easing.