Zone refined niobium bar.
An example of the element Niobium

This is a piece of niobium bar that was zone refined using an electron beam-the metal is heated equally around its circumference in a small band, melting one short area of the bar, and the molten zone is moved along the bar to segregate impurities to one end. Imagine shaking up a container with oil and water, and letting the less-dense oil float up to the top- it is a similar effect, though not necessarily because of density. This is an excellent process for increasing the purity of metals and some other materials, except that it is quite expensive--it requires a high vacuum chamber which must be "baked out" before use to remove moisture and other volatile chemicals, and then the zone refining itself is a slow process which is very delicate and tricky to do just right, resulting in a very small amount of finished material. This sample was zone refined along a short length of it, and was intended to be sliced lengthwise, polished, and etched in a special mixture of acids to view the difference in internal microstructure between the raw bar and the zone-refined section, though this piece was not actually prepared except for the initial slices.