What Is the Crystal Structure of Aquamarine?

Types

• In shade, it can vary from very pale shades--almost colorless--to deeper blues. It is usually a transparent or translucent stone but sometimes exhibits chatoyancy--the presence of stars or cat&'s eyes in the stone. It tends to form hexagonal crystals when allowed to grow unimpeded.

Features

• Like emerald, aquamarine is one form of the silicate minerals known as beryls. Beryl contains the element beryllium: specifically, it is a beryllium-aluminum-silicate mineral. Beryl is a cyclosilicate or "ring silicate": the silicate tetrahedra that make up its basic structure are linked to form rings. Beryl is a relatively common mineral. The gemstone forms of beryl are highly sought after.

Composition

• The chemical composition of aquamarine is Be3Al2(Si6O18). It differs from emerald in that its blue color comes from trace amounts of iron impurities in the beryl structure rather than the molecules of chromium and iron, which give emeralds their green coloration. The relative proportions of Fe2+ ions to Fe3+ ions present in the stone will affect the color, as will the location of the iron inside the beryl crystal.

Impurities in the Crystal Structure

• The silicate rings that make up the aquamarine&'s structure and the way they are connected outline minuscule channels in the crystal. These channels are microscopic, but despite their small size they can contain water and color-influencing substances such as iron. Where there is more Fe3+, the color of the aquamarine crystal will tend toward green, and where there is more Fe2+, the color will tend toward blue. Iron can also substitute for aluminum in certain sites, tending to create a yellow color.

Artificially Altered Crystal Structures

• It is not uncommon for gemstones to be treated in some way to enhance their color. Aquamarine is sometimes heat-treated to alter the crustal structure slightly. This treatment changes the relative levels of ferrous ions Fe3+ and Fe2+, which creates a stronger blue shade with less green coloration. The process can be reversed by irradiation. These shades are more highly prized in jewelry and therefore fetch higher prices.