The more a specimen is magnified (in other words, the higher the power used on the microscope), the less depth of field you'll be able to keep in focus. This is a consequence of how lenses work, and there's very little that can be done about it. What this means to you as a micro-mineral collector is that specimens that have areas of interest about ¼ of an inch across will work best, as they will allow the microscope to reveal very tiny details, yet will not force you to use the higher powers such as 60x and 200x, which are quite limited in depth of field. Of course, if you find a specimen where the detail is relatively flat, such as a fossil or a dendritic crystal pattern, then by all means look for even smaller specimens.