Aquamarine is valued for its serene blue-to-cyan color, often evoking the clarity of seawater. It is a specific variety of beryl, a mineral family that also includes emerald and morganite. Its durability, ranking between 7.5 and 8 on the Mohs scale of hardness, makes it a popular choice for all types of jewelry.
Chemical Identity and Structure
Aquamarine is defined by its membership in the beryl group of minerals. Its chemical formula is Be3Al2Si6O18, identifying it as a beryllium aluminum silicate.
The structure is built around four primary elements: Beryllium (Be), Aluminum (Al), Silicon (Si), and Oxygen (O). Silicon and Oxygen link together to form silicate rings, which create the backbone of the crystal structure. These rings are stacked into channels along the length of the crystal.
Aluminum atoms occupy sites that connect the silicate rings, providing stability to the framework. The atoms are arranged in a rigid, hexagonal crystal system, characteristic of the beryl family. This structure allows the crystal to grow into the long, prismatic columns often seen in raw specimens.
Impurities That Create the Color
Trace elements within the crystal structure determine the specific variety and color of beryl. Aquamarine’s signature blue hue is primarily caused by minute amounts of iron impurities incorporated into the crystal lattice. This differs from emerald, which gets its green color from trace amounts of chromium and sometimes vanadium.
The blue color is caused by ferrous iron (Fe2+) ions substituting for aluminum ions (Al3+) within the beryl structure. The iron’s concentration and oxidation state determine the exact shade, ranging from pale blue to saturated blue-green. When both ferrous (Fe2+) and ferric iron (Fe3+) are present, the result is a greenish-blue color.
Heat treatment is used to enhance the color of aquamarine. When greenish aquamarine is exposed to low heat (700 to 800 degrees Fahrenheit), the ferric iron converts to ferrous iron. This change in oxidation state removes the greenish tint, resulting in a purer blue color. Nearly all pure blue aquamarine on the market has undergone this enhancement.
Geological Formation and Occurrence
Aquamarine formation requires a specific geological environment for its unique chemical structure to crystallize. The most common host rock for gem-quality aquamarine is pegmatite, a coarse-grained igneous rock. Pegmatites are the final, water-rich liquid remaining after a large body of magma has mostly crystallized.
This late-stage fluid is highly concentrated with volatile elements and rare minerals, including the Beryllium needed for beryl formation. As this fluid cools slowly, elements combine and grow into large, well-formed crystals within voids or cracks. This slow process allows the characteristic hexagonal crystals of aquamarine to develop to significant sizes.
While pegmatites are the main source, aquamarine can also form in veins within metamorphic rocks, where hot, mineral-rich hydrothermal fluids circulate. Major global sources include Brazil, Pakistan, Zambia, and Madagascar. The presence of Beryllium-rich magma and the correct iron trace elements dictates where these blue gems occur.