Aquamarine is colored by trace amounts of iron. Most gem aquamarines have been heat treated to produce the popular blue-green colors from less desirable yellow or pale stones. The leading producer of aquamarines is the country of Brazil, which has many mines. Pakistan, as well as many U.S. localities, produce wonderful specimens as well. Recently, a new mine in China has produced large numbers of excellent flat (stubby) hexagonal crystals, for a fraction of the price of those beautiful Pakistan specimens.
Aquamarine is sometimes found in huge crystals (unlike emerald). It is also known in a glassy form, often without showing crystal form, and sometimes with many holes or other odd shapes. Other times a large cluster of prismatic crystals can be seen to all have the same alignment - a sign that it is really a single crystal with many faces.
Aquamarine often forms perfect, flawless crystals - also unlike Emerald for which flawless natural crystals are extremely rare.