Aquamarine (#23)

A 60 gram gem doubly terminated etched blue crystal.
South America Jaqueto, Medieros Netos County, Bahia, Brazil 1 ¾" x 1" x 1"

 

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Aquamarine - is it a mineral or a color? Well, aquamarine is certainly a color, but it is named for the mineral. Aquamarine is a variety of beryl, and it is a semi-precious gem. In fact, several varieties of beryl (defined by their colors) are gems: emerald, morganite, heliodore, golden beryl, and red beryl, as well as aquamarine. The formula of beryl is Be3Al2Si6O18. Trace or minor nonessential elements cause the color.

Aquamarine owes its pale blue color to what are called crystal field effects involving trace amounts of nonessential iron in the specimen. Here's what that means:

If you have taken any chemistry course, you know about d electron orbitals, and that the transition elements (Sc through Zn) have electrons in those orbitals. If a transition element, say Fe, were isolated from other atoms, all five d electron orbitals would have the same energy. However, in the presence of oxygen atoms around the Fe, some of these orbitals are closer to the 0 atoms than others, and so they have different energies. Nature dictates that electrons fill the orbitals in order from lowest to highest energy. Fe2+, the cause of color in aquamarine, has 6 d electrons distributed like this: