In The Elder Scrolls IV: Oblivion, you cannot directly empty a soul gem once it is filled by simply dropping or using it; dropping a filled soul gem on the ground and picking it up again will not empty it
. The game does not provide a standard in-game method to "empty" a filled soul gem back into an empty state. To get empty soul gems, including grand soul gems, you must find or acquire them as empty items. These can be found in specific locations, purchased from merchants, or obtained through lockpicking or pickpocketing in places like the Mages Guild or Arcane University
. For example, empty grand soul gems are rare but can be found in locations such as Squandered Mine and Crayfish Cave after certain quests are activated
. If you want to reuse soul gems for enchanting or recharging, you need empty ones, so managing your supply involves collecting empty soul gems rather than emptying filled ones. Alternatively, some players use the reusable Azura's Star, which acts as an infinite grand soul gem
. In summary, there is no way to empty a filled soul gem in Oblivion; you must obtain empty soul gems separately to use for enchanting or other purposes
