Topaz

Topaz is a type of Silicate - to be precise, an Aluminium and Flourine Silicate.

In it's pure state, it is colourless and transparent - but impurities usually colour it Red, Yellow, Gray, Orange or Blue-Brown.

With a Hardness of 8.0 on the Mohs Hardness Scale, it is quite a hard Gem.

Some Examples are shown below.