Topaz is an aluminium silicate containing fluorine and hydroxyl. Purely, it is colourless (white) ; it is the impurities which cause the variations of colour. “Topaz ranges from colourless to yellow, orange, red-brown, light to dark-blue, pink to red, violet and light-green.” Furthermore, Topaz is distinguished by its hardness and its perfect cleavage. This fine stone is found in Brazil, Afghanistan, Australia, Myanmar (Burma), China, Germany, Japan, Madagascar, Mexico, Namibia, Nigeria, Pakistan, Russia, Zimbabwe, Sri Lanka, Ukraine and in the USA.

Blue topaz are very rarely in the nature (deposits in Northern Ireland and UK), and there are only in a very pale blue colour. The other shades of blue result from techniques (irradiation and heating).

Sources : http://www.gemselect.com/gem-info/topaz/topaz-info.php
and http://www.gemselect.com/other-info/topaz-colors.php