The Round Jewel is a quest item found within Oracle of Seasons. It is one of the four collectible Jewels, along with the X-Shaped Jewel, Square Jewel, and the Pyramid Jewel. Link must collect all four of these jewels in order to gain entrance to the Tarm Ruins.
The Round Jewel inside of a Cave at the southeast corner of Holodrum Plain. The jewel is given to Link by an Old Man, but can only be acquired after Link has completed the fifth dungeon of the game, Unicorn's Cave. Link will need to use either the Zora's Flippers or have Dimitri as an Animal Companion to reach this cave.