This sample of uraninite contains some about 10-9 grammes of actinium at any one time. Image adapted with permission from Prof James Marshall"s (U. North Texas, USA) Walking Tour of the elements CD.
Actinium is dangerously radioactive. The chemical behaviour of actinium is similar to that of the rare earths, particularly lanthanum. It is found naturally in uranium ores and actinium is 150 times more radioactive than radium.
Here is a brief summary of the isolation of actinium.