Aquamarine

Aquamarine is the blue variety of beryl - the same mineral species that includes emerald (green), morganite (pink), and heliodor (yellow). It forms in granitic pegmatites, the coarse-grained igneous rocks that crystallize from the last volatile-rich dregs of cooling magma.

The blue color comes from trace amounts of iron (Fe²⁺) substituting for aluminum in the crystal structure. Unlike emerald, which is colored by chromium and typically riddled with inclusions, aquamarine tends to form in cleaner geological environments, resulting in crystals of exceptional clarity. Some aquamarine crystals from Brazil have grown to extraordinary sizes - the Dom Pedro, the world's largest cut aquamarine, weighs 10,363 carats (over 2 kg) and stands in the Smithsonian.

Most aquamarine on the market has been heat-treated to remove greenish tints, shifting the color toward a purer blue. This treatment involves heating the stone to about 400-450 degrees Celsius, which changes the oxidation state of the iron. The treatment is permanent, stable, and universally accepted in the gem trade.

Aquamarine's combination of blue color, excellent transparency, and hexagonal crystal habit is distinctive. At 7.5 Mohs, it's hard and durable. It lacks cleavage (unlike topaz, which it sometimes resembles) and has a vitreous luster.

Distinguish from blue topaz (slightly higher specific gravity, perfect cleavage), blue tourmaline/indicolite (triangular cross-section), and blue glass (warm to touch, may contain bubbles). Aquamarine often contains characteristic 'rain' inclusions - tiny parallel tubes that are diagnostic of beryl. Viewed down the crystal axis, beryl shows a distinctive hexagonal shape.