[Daishichimaru]

As long as you have a material assigned somewhere you can safely delete it from the editor (note that you must select to ONLY delete the editor's copy) and retrieve it at any later time by retrieving a material from the scene. As a side note, to keep things manageable you can always make small geospheres, turn off the "renderable" option in object properties, apply materials, and hide/freeze them out of the way. Name them something along the lines of "Material Holder: Materialname" and you won't have to worry about losing that hard-won material. Lots easier than flooding your library with things you won't need more than once.