Aquamarine

About a billion years ago, in what is now the Brazilian state of Rondônia, a body of granite was crystallising underground at Massangana. Its last, fluid-rich part gathered near the top and set as pegmatite, with pockets of open space inside. In one of those pockets a crystal of beryl began to grow. Beryllium, aluminium, silicon and oxygen came out of the hot fluid filling the pocket and locked together in the six-sided pattern of beryl, and the crystal built outward, one zone around the next.

Zircon in that granite dates it to between about 1,026 and 996 million years ago. At about the same time, continents elsewhere were colliding along a belt that circled the globe, joining into the supercontinent geologists call Rodinia.

Iron came in with the other atoms and took the place of some of the aluminium. Sodium and water slipped into the open channels that run through beryl and kept the charges balanced. That iron gave the crystal its blue.

The supply did not stay steady. As more or less iron and sodium reached the crystal, each new zone came out a slightly different blue, and Massangana aquamarine runs from light blue to medium blue in rings. Iron in another form, Fe³⁺, brings yellow, and where it was strong a crystal leaned green. Seen from different directions, the same crystal also shows a slightly different shade.

Now and then a growing face closed over a drop of the fluid around it. The drop stayed inside, sealed with a small bubble of vapour.

In 2020 Ingrid de Souza Hoyer, Valmir da Silva Souza and Beatriz Debowski studied aquamarine from the Massangana workings. They set the crystals on a heating stage under a microscope and warmed them until the bubble in each inclusion disappeared. That happened between 243 and 315 °C. The fluid was water with a little salt in it, and those temperatures fit the last stage of a pegmatite pocket as it cooled.

When the pocket had cooled, the crystal stayed shut in the rock until small-scale miners, garimpeiros, dug it out. Most aquamarine sold today is heated after that. Heat weakens the yellow-green part of the colour and leaves a cleaner blue. It is still beryl afterwards.

For a bead, a piece of the crystal is rounded, drilled and polished. In a larger bead, light crosses more beryl on its way through, so a bigger bead of the same material can look a deeper blue.

On this strand, aquamarine beads sit beside colourless clear quartz. Light goes straight through both, and the blue deepens where it crosses more of a bead. That blue is the iron the crystal took in as it grew, with some of its yellow cleared by heat. At about 7.5 to 8 on the Mohs scale, aquamarine resists scratches well, and a hard knock can still chip it.

The aquamarine in our strands also comes from Brazil, where pegmatites of different ages hold it, and it has been heated. The clear quartz beside it is untreated. Each strand comes with a Stone Origin Record.

One way to read this: aquamarine is the blue variety of beryl. Pure beryl is colourless; the blue comes from iron that entered the crystal as it grew, most often in granite pegmatites. Aquamarine crystals are known for being large and unusually clean, and the colour sits in a stone you can see straight through. The colour is definite, and the stone stays transparent.

Let it through. The meaning is yours.