8.0 on the Mohs scale.

Brazil, U.S., Sri Lanka (Ceylon), Myanmar (Burma), former USSR, Australia, Pakistan, Mexico, Tasmania, Japan, Africa. Topaz and citrine are the birthstones of the month of November.

Yellow topaz is sometimes called "precious topaz" to distinguish it from the names "gold" and "madeiria topaz", which are in fact citrine (quartz). Yellow topaz is sometimes heat treated to make it look pink. The term "imperial topaz" is used by ethical jewelers to refer to topaz that is reddish orange of medium tone and higher saturation.

Yellow/Orange topaz is occasionally irradiated to intensify color. Pink/Red topaz is usually heat treated chromium bearing pinkish-brown to orange stones. Brown topaz is not enhanced. Green and blue topaz are usually irradiated to produce the desired color.

More information on gemstone enhancements.