Because all lanthanides are close in size, the rare earth elements have always been hard to separate. Even with eons of geological time, separation of the lanthanides in nature has only rarely gone farther than the separation between light versus heavy lanthanides, otherwise known as the cerium and yttrium earths. This geochemical separation is shown in the first two rare earths that were discovered. These were yttria in 1794 and ceria in 1803. When first found, each was a mixture of all of the rare earths. Large ore bodies of the cerium earths can be found around the world and are being exploited. Ore bodies for yttrium are rarer. Also, they are usually smaller and less concentrated.