As a member of the beryl mineral family, aquamarine has a chemical composition of Be₃Al₂Si₆O₁₈. Pure beryl is colorless. The greenish blue to blue color of aquamarine is caused by trace amounts of iron.

The iron can occur in two oxidation states. Ferrous iron (Fe²⁺) is responsible for blue color, and ferric iron (Fe³⁺) is responsible for yellow color (which combines with the blue color to produce greenish blue).

Most natural aquamarine has a greenish blue color. Gentle heat treatment can remove the yellow color and transform the gem’s color to a more pure and more valuable blue.

Greenish yellow and other colors of beryl can be altered to colors within the aquamarine range by heat treatment. [3] Most of the aquamarine offered for sale in today’s market has received heat treatment. The color improvements achieved by heat treatment are thought to be permanent and undetectable by current methods of testing.

Any time a gem has been heated, irradiated, dyed, or undergone other treatments to improve its color, durability or appearance, the seller should disclose that information to the buyer. This information is important to many buyers – even to buyers of low-cost items.