Aquamarine grows in granite pegmatites, formations created when the last fluids of a crystallizing magma solidify into coarse-grained granite. This occurs far below the earth's surface, at depths up to 20 miles down. As hot mineral-rich solutions move through fractures in the granite, beryl may crystallize if the chemical conditions are right. An aquamarine forms when beryl contains trace amounts of iron that tint it light blue.
The geological processes that create aquamarines take place over millions of years. First, magma from deep in the earth's mantle rises toward the surface. This molten rock carries dissolved minerals like silica, aluminum, sodium, lithium, cesium, and beryllium – the chemical components of beryl. As the magma nears the surface and starts to cool, the first minerals to crystallize are darker silicates like pyroxene, amphibole, and mica.
Once most of these mafic minerals have crystallized, the remaining magma is enriched in silica, beryllium, and alkali metals. This chemically evolved magma has a high viscosity and struggles to erupt onto the earth's surface. Instead, it stalls in subsurface intrusions like dikes and sills. The last vestiges of this silica-rich magma crystallize into granite pegmatites filled with minerals like quartz, feldspar, and beryl.
The pegmatites form large crystal cavities tens of feet long and several feet wide. As hot aqueous solutions containing dissolved beryllium, aluminum, and silica move through the pegmatite, they deposit beryl and other minerals on the cavity walls. An aquamarine begins as a seed crystal that gradually grows larger over thousands to millions of years as more beryl accumulates around it.
The size of the seed crystal and the chemical composition of the mineralizing solution control the ultimate dimensions, color, and clarity of the finished gem. An aquamarine grows best when it has plenty of open cavity space and a steady supply of nutrient-rich fluids. The width of a single aquamarine crystal can exceed 3 feet!
While the pegmatite is still deep in the earth, aquamarines form as rough, opaque crystals. Tectonic forces eventually uplift the host pegmatite toward the surface. The aquamarine-bearing pegmatites are exposed by erosion and weathering of the overlying rock.
Miners locate aquamarine pegmatites and extract gem-quality crystals. The mined aquamarine crystals are sorted based on color, clarity, and size. The most flawless, vividly colored chunks are cut into faceted gemstones that maximize their brilliance. Pieces with defects are carved into beads or other ornamental objects. Small or pale crystals are usually cut en masse into cabochons.
From its beginnings far underground to the jeweler's display case, the aquamarine takes an incredible journey. These luminous blue gems captivate us with their elegance and subtle oceanic palette. Their meditative hue and crystalline structure are windows into the awesome forces that shape our dynamic planet.
