AQUAMARINE

Aquamarine is the blue variety of Beryl, named for it's colour. It occurs in a range of shades, from light blue to deep sky-blue, some stones having a sea-green tone.
Its colour is due to the addition of ferrous iron (Fe²⁺) to the beryllium aluminium silicate during the crystals growth.
While large crystals are not uncommon, clean stones with a good colour are harder to find, with most commercially available Aquamarine being heat treated to enhance colour.