Pure topaz is transparent but is usually tinted by impurities; typical topaz is wine or straw-yellow. They may also be white, grey, green, blue, pink or reddish-yellow and transparent or translucent.
The colour in which the topaz is most often found is yellow, and that is the colour in which it occurs in one of the major German sources, the Schneckenstein (a topaz-bearing rock said to resemble a snail) in Saxony. In the 18th century, it was mined there for over 60 years. However, most of the crystals were hardly a centimetre in diameter. You had to go to Siberia or Brazil to find crystals as large as your fist.
When heated, yellow topaz often becomes reddish-pink. The colour change upon heating was first discovered by a Parisian jeweller around 1750. In particular the yellow Topaz of Brazil has been known to be treated frequently. Only stones of a brown-yellow colour yield the pink; the pale yellow ones usually turn white. The pink colour is stable. Topaz can also be irradiated, turning the stone blue, ranging from a light pure colour to very dark almost electric blue. A recent trend in jewellery is the manufacture of topaz specimens that display iridescent colours, by applying a thin layer of titanium oxide via physical vapour deposition, this stone is then sold as 'mystic topaz'. We have not seen it and can't see us ever touching the stuff.