Aquamarine

Aquamarine is a beryl, the same beryllium aluminium silicate species as emerald, coloured a pale blue to blue-green by traces of iron rather than the chromium or vanadium that makes emerald green. It is hard, around 7.5 to 8, and unlike emerald it usually grows clean, so large flawless stones are common and the colour rather than the clarity sets the value; the most prized tone is a pure blue, and much commercial material is heat-treated to remove a greenish or yellow cast. The name is Latin, aqua marina, sea water, and the stone was a sailors talisman in the Roman and later European tradition, carried against drowning and for calm passage. Modern practice makes it the March birthstone and, in the twentieth-century crystal literature, a stone of the throat centre and of clear speech; those associations are recent and rest on the colour and the name rather than on any older source.

The eleventh-century French bishop Marbode of Rennes, whose verse lapidary De Lapidibus was the most copied gem-text of the medieval West, recommends beryl, the family aquamarine belongs to, as a stone that steadies the tongue in argument and gives an advocate command of his case, a specifically forensic virtue distinct from the sailor's talisman recorded above. Pliny the Elder likewise reports that beryls were commonly cut smooth and cylindrical rather than faceted, since the ancients valued an unbroken length of colour over the play of light later cutting would bring out, and that engraved beryls were exchanged between lovers to bind their affection.

Aquamarine is blue beryl, coloured by iron, and two things about it are worth separating: an old association that is genuinely old, and a modern colour that is not.

The association is with the sea and with safe passage on it. The name is Latin, water of the sea, and the connection runs through the classical and medieval lapidaries, where the stone is carried by those going on a voyage. That is a straightforward case of a colour suggesting a meaning, and it is unusually consistent across the sources.

The modern colour is another matter. Natural aquamarine as it comes out of the ground is very often green or greenish blue. Heating, at a few hundred degrees, converts the iron responsible for the green component and leaves the pure blue that the market wants. The treatment is stable, permanent, undetectable by any routine test, and applied to the overwhelming majority of the material sold. Untreated green-blue aquamarine has recently acquired a following of its own precisely because it is what the mineral actually does.

The principal source is Minas Gerais in Brazil, where beryl pegmatites have produced crystals of remarkable size. The Dom Pedro aquamarine, a carved obelisk of over ten thousand carats now in the Smithsonian, was cut from a crystal from that region, and it is the largest cut aquamarine known.

Because beryl grows in long clean prisms, aquamarine is one of the few gems routinely available in large, flawless, well-matched stones, which has shaped how it is used in jewellery.