Available: From the point where you join the College.
Note: This quest/task DOES NOT show up in your quest list, not even under miscellaneous quests.
Just like the previous Radiant Quest, speak to Sergius and he wants some Soul Gems . He wants a [random number] of a [random] Soul Gem. When you find in your journey the amount of Soul Gems he requests for, bring them to him and he’ll give you gold in exchange.
Note: He will NOT accept stolen Soul Gems, but you can find soul gems just laying around the college fairly often (there’s loads in the Arch-Mage’s room).