Aquamarine

Aquamarine is a mineral composed of Beryllium Aluminium and Silica - with the blue colouring attributed to trace amounts of Iron.
The mineral "Beryl" in fact gives 2 different closely related gemstones - Aquamarine with the iron impurities and Emerald with Chromium impurities.
 
With a Hardness on the Mohs scale of 7.5 to 8 it is relatively hard and long lasting although it can be brittle.
 
Some examples of Aquamarine are shown below.