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.