Topaz is allochromatic, which means that its colour is caused by impurity elements or defects in its crystal structure rather than by an element of its basic chemical composition. The element chromium causes natural pink, red and violet-to-purple colours in topaz. Imperfections at the atomic level in topaz crystal structure can cause yellow, brown and blue colour. Brown is a common topaz colour, and the gem is sometimes mistakenly called “smoky quartz”.
Topaz actually has an exceptionally wide colour range that, besides brown, includes various tones and saturations of blue, green, yellow, orange, red, pink, and purple. Colourless topaz is plentiful, and is often treated to give it a blue colour.
The colour varieties are often identified simply by hue name—blue topaz, pink topaz and so forth—but there are also a couple of special trade names. Imperial topaz is a medium reddish orange to orange-red. This is one of the gem’s most expensive colours. Sherry topaz—named after the sherry wine—is a yellowish brown or brownish yellow to orange. Stones in this colour range are often called precious topaz to help distinguish them from the similarly coloured but less expensive citrine and smoky quartz.
Topaz is also pleochroic, meaning that the gem can show different colours in different crystal directions.