A thunderegg is one of the more dazzling artifacts that life as a geologist can turn up; indeed, they are so aesthetically pleasing that they have become a sought-after commodity for people with no real interest in rock, but plenty of interest in beautiful artifacts. You could pay $60 or more for a thunderegg, so it is invaluable that you know how to care for it. Granted, you could simply throw it in a rock tumbler, but you could do a more thorough job by applying the personal touch.
Use a small hammer and chisel to remove any imperfections which may be protruding from the surface of your thunderegg. Any attempt to polish the surface will be constantly sabotaged by and bumps or grooves, so you should attempt to remove them.
Rub the thunderegg against a slab of concrete in order to file down the surface of the rock. Again, this is to create an even surface, which you can gradually perfect and then polish.
Sand the rock into the final shape you would like it to be, complete with rounded edges, by rubbing it with coarse sandpaper -- around 50-grain sandpaper should be enough to smooth and shape your thunderegg.
Use gradually finer sandpaper to improve the surface of your thunderegg. Work on the rock with 150-grain, then 600-grain, before finishing the job with a layer of 1500-grain exceedingly fine-grit sandpaper. At this stage you should have a well-rounded, smooth thunderegg.
Apply a small amount of polish to a leather cloth and buff your thunderegg until you are happy with the even, lustrous shine.
This is a time-consuming project and the thunderegg will smooth, polish and shine in gradual phases. Be patient and the end product will be spectacular.