Aquamarine

Aquamarine is beryl coloured by iron. It is the same mineral as emerald and is almost its opposite as a gemstone: it grows in pegmatites, which are calm environments by geological standards, and the crystals are correspondingly large and clean. Metre-scale flawless crystals have been recovered, which no emerald deposit has produced.

The colour depends on the oxidation state of the iron. Ferrous iron gives blue, ferric gives yellow, and a stone containing both is the greenish blue most aquamarine comes out of the ground as. Heating to around 400 °C converts the ferric to ferrous and leaves a pure blue, which is what the market prefers and what most aquamarine on sale has had done to it. The change is stable and permanent.

Pale blue to greenish blue hexagonal prisms, often with a flat termination and vertical striations along the prism. Distinguished from blue topaz by a lower refractive index and from blue zircon by being far less dense — zircon is nearly twice the density.

Strongly pleochroic: the same stone looks near-colourless down one axis and blue down another, which is why cutting orientation matters so much to the finished colour.

Gemstone, and the large clean crystals make it one of the few in which very big cut stones are ordinary rather than remarkable. Faceted, carved, and used in objects where a large clear blue stone is wanted.

Heat treatment is routine, permanent and disclosed. Irradiated and dyed imitations exist and are a different matter.

From the Latin aqua marina, sea water, recorded in this sense from the seventeenth century. It carried a sailor's reputation as a protective stone, which is the usual pattern of a name describing a colour and a folklore growing out of the name.