Aquamarine is the blue to greenish blue variety of the mineral beryl, colored by iron. It grows into large, clean crystals, so it is one of the few gems readily available in big, eye-clean sizes. Most aquamarine on the market has been gently heated to remove green tones, and it is the modern birthstone for March.
What aquamarine is
Beryl is a beryllium aluminum silicate. In its pure form it is colorless (goshenite). Trace elements give the colored varieties their names: chromium or vanadium makes emerald, manganese makes pink morganite, and iron makes aquamarine and yellow heliodor. The name aquamarine comes from the Latin for "seawater."
Iron sits in two forms in beryl. Ferrous iron (Fe2+) is linked to the blue color, while ferric iron (Fe3+) adds yellow. A mix of the two gives the blue-green color common in rough crystals. Heat converts some ferric iron to ferrous, which removes the yellow component and leaves a purer blue. The color in gemstones page explains how trace elements like iron act as chromophores.
Aquamarine has a hardness of 7.5 to 8 on the Mohs scale, a refractive index of about 1.567 to 1.590 and a specific gravity around 2.68 to 2.74 for most stones. It shows noticeable dichroism: one direction looks nearly colorless or pale blue, the other a deeper blue. Cutters orient the table to show the stronger blue.
Color and the Santa Maria name
Aquamarine color ranges from very pale blue to a deep, slightly greenish blue. Most stones are light. The trade values the most saturated, pure medium blue with no gray or green.
"Santa Maria" is the best-known color term. It comes from the Santa Maria de Itabira mine in Minas Gerais, Brazil, which produced intensely blue crystals in the mid twentieth century. The name stuck as a description of top color, and stones from Mozambique and other African sources with similar depth have been sold as "Santa Maria Africana." Like "Siberian" amethyst, the term now describes a grade rather than a guaranteed origin.
"Espirito Santo" was another Brazilian name used for fine, slightly lighter blue. Light stones are sometimes simply described as "sky blue."
Aquamarine color is subtle in small sizes. A stone needs some depth or size to show strong blue, which is one reason large stones are prized.
Heat treatment
Most aquamarine is heated at relatively low temperatures, often a few hundred degrees Celsius, to remove the green or yellow tint. The result is stable and permanent and cannot be detected by standard gemological tests in most cases. Because the treatment is so common and undetectable, the trade generally assumes aquamarine may be heated. You can read about disclosure norms in gemstone treatments explained.
Some green beryl is sold as "green aquamarine." Beryl collectors and labs differ on whether greenish material should carry the aquamarine name. Maxixe beryl, a deep blue beryl colored by irradiation, fades in light and is not aquamarine.
Sources
Aquamarine forms mainly in granite pegmatites, coarse-grained igneous rocks that crystallize from the last, water-rich fluids of a cooling magma. These pockets allow large, clean crystals to grow. The igneous rocks and gemstones page describes pegmatites in more detail.
Brazil
Brazil, especially Minas Gerais, has long been the classic source. The state's pegmatite districts around Teofilo Otoni and Governador Valadares have produced huge crystals for over a century. In 1910 a single crystal weighing about 110 kilograms was found at Marambaia. Read more in gems of Minas Gerais.
Africa
Nigeria produces deep blue aquamarine from pegmatites in the central part of the country. Madagascar has supplied fine stones, some with a strong blue in smaller sizes. Mozambique, Zambia, Kenya and Namibia have also produced gem material.
Asia and elsewhere
Pakistan's northern mountains yield well-formed aquamarine crystals on matrix, prized by mineral collectors. Afghanistan, Russia (the Urals and Siberia), Sri Lanka and the United States (Colorado, where aquamarine is the state gem, and California) have also produced it.
Large crystals and the Dom Pedro
Aquamarine is one of the few gems where very large faceted stones and carvings exist. Clean crystals weighing many kilograms are not unusual in pegmatite pockets.
The most famous is the Dom Pedro aquamarine, an obelisk-shaped carving cut in Idar-Oberstein, Germany, by the gem artist Bernd Munsteiner from a crystal found in Brazil in the 1980s. The finished piece weighs about 10,363 carats and stands roughly 35 centimeters tall. It was donated to the Smithsonian's National Museum of Natural History in 2011, where it is on public display. The Idar-Oberstein gem cutting page has more on Munsteiner's fantasy cuts.
Clarity, cut and value
Aquamarine is a Type I colored stone, meaning it is often eye-clean. Buyers expect clean stones and pay less for visible inclusions. Inclusions that do appear include long, hollow growth tubes and fluid inclusions; when tubes are dense and parallel, they can produce a cat's-eye effect in a cabochon.
Emerald cuts, ovals and cushions are popular because they show the color well in light stones. Value rises with depth of blue, cleanness and size, and the price per carat can climb steeply for intensely colored stones over a few carats.
Wear and care
At Mohs 7.5 to 8, aquamarine handles daily wear in rings and pendants. It has poor cleavage and can chip if struck sharply at the girdle or corners. Protective settings help for rings worn every day.
Clean aquamarine with warm soapy water and a soft brush. Ultrasonic cleaning is generally safe for clean stones but risky for those with fractures or growth tubes. Avoid steam and sudden temperature changes. See the jewelry cleaning guide.
History and birthstone
Ancient sailors were said to carry aquamarine as a charm for safe passage at sea, a belief tied to its seawater color. Medieval Europeans used beryl crystals for scrying and as early lenses. Such stories are folklore, and the gemstone meanings and folklore page gathers others.
Aquamarine is the modern March birthstone, sharing the month with bloodstone on the birthstones by month list. It is also a traditional gift for the nineteenth wedding anniversary.
Frequently asked questions
Is aquamarine always heat treated?
Most aquamarine is heated to remove green, and the treatment is usually undetectable. Some stones come out of the ground blue and are sold unheated.
What is the best aquamarine color?
The most valued color is a saturated, pure medium to medium-dark blue with no green or gray, often called Santa Maria color.
Is aquamarine good for an engagement ring?
Aquamarine's hardness of 7.5 to 8 suits a ring, but it can chip if knocked. A protective setting and some care help it last.
How is aquamarine related to emerald?
Both are varieties of beryl. Emerald gets its green from chromium or vanadium, while aquamarine gets its blue from iron.