Pure topaz is colorless and transparent but is usually tinted by impurities; typical topaz is wine, yellow, pale gray, reddish-orange, or blue brown. Typically, colorless, gray or pale yellow and blue material is heat treated and irradiated to produce a more desired darker blue. In this case, it has produced the “Sky Blue” color.
NOTE: The mark you can see on the stone is just from the bluetac holding the stone up during the photo. It does not exist on the stone
This Topaz is natural. It is irradiated to bring out the blue color.