Beryllium crystal. Coloured scanning electron micrograph (SEM) of cleavage in a beryllium (symbol Be) crystal. Cleavage is splitting, or tendency of a crystal to split, along definite crystalline planes to produce a smooth surface. Here, 2 types of cleavage are seen: basal cleavage (centre and upper frame) where the crystal can be peeled off in flat sheets; and prismatic cleavage (bottom frame) where the crystal can be broken off as thin prisms. Beryllium, a group 2A chemical element, is used in nuclear reactor reflectors. This crystal contains 0. 17 mole percent of copper. Magnification: x126 at 6x7cm size.