In the early stages of crystallization, the ions that form high-temperature minerals are depleted from the melt. Rare ions that do not participate in the crystallization of common rock-forming minerals become concentrated in the melt and in the excluded water. These ions can form the rare minerals that are often found in pegmatites. Examples are small ions such as lithium and beryllium that form spodumene and beryl; or large ions such as tantalum and niobium that form minerals such as tantalite and niobite. Rare elements concentrated in large crystals make pegmatite a potential source of valuable ore.