You can find Grand Soul Gems in Oblivion at several guaranteed and possible locations, as well as from merchants. Here are the best places to find them:
Guaranteed Locations with Grand Soul Gems
- Arcane University :
- Three Grand Soul Gems inside display cases in the Arch-Mage's Lobby (behind a Very Hard Lock)
- One in the Mage's Quarters (also behind a Very Hard Lock)
- Chorrol Mages Guild :
- Two Grand Soul Gems in a display case on the main floor (Very Hard Lock)
- Three more in another display case near the front door
- Squandered Mine : One Grand Soul Gem on the lower level
- Drakelowe : One Grand Soul Gem in the basement (secret fireplace entrance)
- Nornal Ruins : One Grand Soul Gem on the ruins west of Drakelowe
- Crayfish Cave : One Grand Soul Gem in the cave south down the river
Possible Locations (May Not Always Have Gems)
- Imperial City Market District : Buy from Aurelinwae and Calindil at the Mystic Emporium
- Mages Guild Halls : Various merchants may have Grand Soul Gems for sale
- Ayleid Ruins : Search Ayleid Casks and containers
- Dagon Shrine Living Quarters : One can spawn in a chest during the Mythic Dawn infiltration
Additional Tips
- Grand Soul Gems start spawning more frequently as your character levels up, typically from level 17 or 20 onwards.
- Waiting for seven days resets merchant inventories, so check back regularly.
- Spellcasting enemies often drop Soul Gems as loot.
- For the Vampire Cure quest, five empty Grand Soul Gems are required, which can be found in Squandered Mine and Crayfish Cave after starting the quest.
- You can also steal or pickpocket Grand Soul Gems from locked display cases in places like the Arcane University and Chorrol Mages Guild.
- Buying from merchants usually yields filled Grand Soul Gems, so be sure to check if they are empty if you need them for enchanting or quest purposes
This comprehensive list should help you reliably find Grand Soul Gems in Oblivion.
