Uranium!

Uranium is more abundant on Earth than mercury, silver, or cadmium. (Were this dense metal [~19 g / cc] to be made more readily available to the general public, it would no doubt find application in nifty paperweights.) Presently, the metal itself is used in radiation shielding due to its high density and high atomic mass. It is also used in aircraft control-surface counterweights, and in armor-penetrating munitions for the military. The vast majority of uranium employed in these applications is "depleted uranium," or uranium in which much of the isotope U-235 has been removed. Where does the U-235 go? It gets turned into nuclear weapons and reactor fuel because it is fissioned by slow neutrons!

Uranium metal should be handled carefully. Small bits have a propensity to catch fire spontaneously in the air. This property, known as pyrophoricity, is common to a number of rare earth metals including the alloy used in cigarette lighter flints. Like lead and mercury, uranium is rather poisonous. Do not eat, drink, or smoke while handling uranium.

In the United States, the NRC regulates uranium. There are all sorts of rules. Businesses and schools without a specific license are restricted to an annual bag limit of 150 pounds of metal, with the stipulation that only 15 pounds may be kept on hand at any one time. If you own any of those little aircraft counterweight blocks, you aren't allowed to scrape off the plating to get at the bare uranium metal and they must be properly labelled. The same goes with radiation shielding made from DU metal. And you aren't allowed to enrich your uranium (unless you have a license), or possess enriched uranium. Stick with the natural or depleted stuff to be on the safe side of the law.