Also in hydrothermal replacement deposits. It is insoluble in acids. Some varieties are heat sensitive. Some varieties can be irradiated to various shades of blue, and this acceptable in the trade. Most blue topaz on the market today is irradiated.
Blue topaz is rare in nature, but easily created from clear material.
There is an abundance on the world market, and very large, flawless stones are easily available. It is available in shades of blue from very light, through sky-blue, and on to almost an inky blue. Overtones of gray are not desirable, and further reduce its value. Red-brown topaz is also common and found in Mexico and Utah, it makes a nice faceted stone, and some don't. Again it is not of high value.