Aquamarine: Properties, Origins, Treatments and Value

Aquamarine is beryl coloured blue by iron, which makes it emerald’s sibling in everything but the two things that make emerald difficult. Iron enters beryl easily and in quiet geological settings, so the crystals grow large, clean and unbroken in the pegmatites that emerald never sees. The result is a gem that is abundant in sizes emerald cannot reach, flawless where emerald is fissured, and priced accordingly: an honest, beautiful, affordable stone that is rarely serious, except at the deep end of its colour.

What it is

Be₃Al₂Si₆O₁₈, the same ring silicate as emerald, crystallising in the hexagonal system as long six-sided prisms that can reach the size of a man. Hardness 7.5 to 8, and because the crystals are rarely fractured the stone is far tougher in practice than emerald, with only an imperfect cleavage to respect. Refractive index 1.577 to 1.583, birefringence near 0.006, specific gravity around 2.70. Pleochroism is distinct, blue down one direction and near-colourless or slightly greenish down the other, so the cutter orients the table across the prism to show the blue, and long crystals become long stones: the emerald cut and long ovals are the natural shapes. Morganite is the pink beryl, heliodor the yellow, goshenite the colourless, and all of them share the same clean habit that emerald does not.

Where the colour comes from

Iron in two states. Ferrous iron, Fe²⁺, gives the blue. Ferric iron, Fe³⁺, gives a yellow that combines with the blue into the greenish tint most rough shows, and gentle heating converts the ferric component and leaves pure blue; the treatment is stable, undetectable and standard. Saturation rises with iron concentration and, because the colour is pale by nature, with the length of the light path, which is why aquamarine is a stone that improves with size and why small stones rarely show much colour at all. There is no fluorescence and no fire to speak of. A separate blue beryl called Maxixe, from a Brazilian mine and later produced by irradiation, is a deep blue from a colour centre that fades in daylight, and it is the one trap in an otherwise honest species.

Where it comes from

Brazil is the reference: Minas Gerais and Espírito Santo have produced aquamarine from pegmatites for two centuries, and the Santa Maria de Itabira mine gave the trade its name for the deepest blue. The Dom Pedro, a carved obelisk of over ten thousand carats in the Smithsonian, was cut from a Brazilian crystal. Mozambique produces fine saturated material from Nampula that trades as Santa Maria Africana; Nigeria, Madagascar and Zambia supply commercial and fine goods. Pakistan’s Shigar valley in the Karakoram gives clean, well-formed crystals prized by collectors as specimens and cut into fine stones. Vietnam, Afghanistan, Namibia, the Urals and Colorado contribute. Origin does not move the price; the name Santa Maria describes a colour, and a laboratory rarely needs to state where an aquamarine was found.

How it is judged

Colour, and almost only colour. On the three axes the market wants a pure blue with no green, and saturation is the whole price: the great majority of aquamarine is pale, a saturated medium blue is uncommon, and the deep Santa Maria blue is rare enough to be priced as a different stone. Tone follows saturation because the colour is inherently light. Then clarity, where the species is expected clean and anything else is discounted; the long hollow tubes that aquamarine sometimes carries are a feature, not a flaw, in a specimen and a fault in a gem. Then cut, which matters more here than in most stones because a pale species depends on a deep, well-proportioned pavilion to hold colour, and a shallow stone goes to water. Then size, where the per-carat curve is gentle by the standards of the trade because large clean crystals are ordinary; only saturated colour in large sizes climbs.

Treatments, and what to demand on paper

Heat is routine, stable, undetectable and standard, and no laboratory attempts to call it; the market prices heated and unheated aquamarine the same because it cannot tell them apart and does not try. The Maxixe-type blue produced by irradiation is the exception: it is a deep blue that fades in light, laboratories can identify it, and it must not be sold as ordinary aquamarine. Synthetic aquamarine exists by hydrothermal growth and is uncommon because the natural stone is cheap. A report is unnecessary for most aquamarine and reasonable for a large stone of saturated colour, where the price justifies confirming identity and excluding Maxixe.

The market

Broad, deep and honest. Commercial aquamarine in pale blue is priced as fine jewellery material by the carat, in sizes that would be historic in any other species. Saturated medium blue in clean stones above ten carats is a real market with real prices. The deep Santa Maria colour in size is a collector’s stone and trades per carat at levels that surprise buyers who know only the pale material. There is no auction record book worth the name, no origin premium and no treatment premium; liquidity is good at the commercial level and thin at the top. Emerald, the same mineral with a different guest, sets prices an order of magnitude higher for stones a tenth the size, and the reason is entirely rarity.

Care

Hardness 7.5 to 8 in an unfractured crystal makes aquamarine one of the more durable stones a jeweller sets. Ultrasonic and steam cleaning are safe on clean stones, daily wear in a ring is reasonable, and there is no oil, filling or heat sensitivity to protect. Keep Maxixe-type stones out of the sun, and keep everything else in it if you like; the colour of a true aquamarine does not move.

Where I stand

Aquamarine is not a stone this office hunts, because it does not need hunting: it is abundant, honest and well served by the ordinary trade. The exception is the deep Santa Maria blue in a large clean stone, which is rare enough to be worth a mandate for a client who wants that colour specifically. For everyone else my advice is the advice for any pale species: buy saturation over size, buy the cut that holds the colour, and do not pay for a name that is applied more often than it is earned.

Questions collectors ask

What is Santa Maria aquamarine?

A trade name for the deepest, most saturated blue the species produces, after the Santa Maria de Itabira mine in Minas Gerais, Brazil, whose stones set the standard. Comparable colour from Mozambique and elsewhere is sold as Santa Maria Africana. Most aquamarine is far paler, and the name is applied more often than it is earned.

Is aquamarine heated?

Routinely. Most rough comes out of the ground greenish from ferric iron, and gentle heating converts that component and leaves pure blue. The treatment is stable, undetectable and accepted as standard, so laboratories do not attempt to call it and the market does not pay for the absence of it.

Why is aquamarine so much cheaper than emerald?

Because it grows large and clean. Beryl coloured by iron crystallises in pegmatites as big, flawless prisms, so clean stones of ten and twenty carats are ordinary and the rarity that drives emerald prices does not exist. Only the deepest saturated blues in large sizes command serious prices.