Sky Blue Topaz is a form of white 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. Sky Blue Topaz is a beautiful shade of pale blue, and it usually has superb clarity, even in large sizes. Blue topaz also comes in a mid-blue; Swiss Blue Topaz and a dark greenish-blue; London Blue Topaz. Sky Blue is the most affordable of the three as it needs less time to treat. As blue topaz is irradiated, it needs a cooling off period to ensure the stones are safe, and this period is shorter than for the other two topazes - around 3 months. All of my blue topaz is treated in the US in line with safety guidelines there, so it is all completely safe.