I just found this while googling. Quite interesting. I did not read all the lessons but it seems easy to do your own restoration on those crystal pickups.
http://philcoradio.com/phpBB2/viewforum.php?f=3&sid=a48ab44c55c2324a90377935aaa29372
marv
:Hi all I have been frustrated by non working late 1940s phonograph pick-ups that are the type made of metal and sealed with rivets and have some black gunk in them. I want to replace this can I remove the rivets and fix it??? If not any modern replacements??? or other ideas??