This gem is part of a very special family of gems called the beryl family.  Other members include emerald, morganite, heliodor, and golden beryl.  Aquamarine was given its name due to its greenish blue color, the color of the sea. Heating removes the greenish component, leaving a more desirable blue color. This process is routinely done because this bluer color is more valued in the marketplace.  Aquamarine is commonly found with few inclusions and therefore when purchasing look for and expect clean material.  

This is a gem whose color usually benefits when the gem is a certain size.  Small gems of under 5 carats often look pale.  Larger sizes of 10 carats and up help to intensify the pastel blue color.  Aquamarine is often found in large crystals so finding large gems of 20, 40, or even 50 carats is not uncommon. Aquamarine from Africa is more intensely colored in the small sizes but tends to have a steely blue color or sometimes a greenish or gray secondary overtone to the blue color. 

The crystal habit is hexagonal and occurs in long crystals that lend themselves to cutting rectangular or long oval shapes.  That is why these are the most common shapes for aquamarine.  Less gem rough is lost using these types of shapes and therefore, less money is lost.  Conversely, unusual shapes, when found, often cost more per carat because of the necessary waste produced in cutting that shape.