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.