Beryl is composed of beryllium aluminum silicate and is the chief ore of the element beryllium. Crystals of beryl in certain unique rocks called pegmatites can be as large as railroad boxcars. These immense crystals are never transparent, but some pegmatites contain smaller, well formed and transparent beryl crystals; these occur in various colors, and each variety used as a gemstone has its own special name. The dark green variety colored by chromium and vanadium is known as emerald. Fine emeralds occur in Brazil, Afghanistan, Russia, Madagascar and many other places, but the best known is probably Colombia. The hexagonal (6-sided) symmetry of the mineral is evident in this Colombian emerald crystal.