The Square Jewel is a quest item found within Oracle of Seasons. It is one of the four collectible Jewels, along with the X-Shaped Jewel, Pyramid Jewel, and the Round Jewel. Link must collect all four of these jewels in order to gain entrance to the Tarm Ruins.
The Square Jewel is found within a Cave at the southeast corner of Spool Swamp. The cave can only be reached during the season of Winter, where the tree in front of it will lose its branches, allowing Link to pass. The actual cave entrance will need to be blown open using a Bomb. In a Linked Game, this cave holds a Treasure Chest that contains 100 Rupees instead of the jewel.
In a Linked Game, the location of the Square Jewel will have changed, now located in a cave in North Horon. Though technically within the North Horon region, the cave that holds the Square Jewel is found beneath Eyeglass Lake. After Link has acquired the power of Summer within his Rod of Seasons, along with Roc's Feather, he will be able to acquire the Square Jewel.
Link will need to use the Tree Stump located in North Horon to change the season to Summer. From there, he can head south to the Eyeglass Lake, where the hot summer season has caused the lake to recede. At the northwest corner of the Lake, Link will find a cave that can be blown up with a Bomb. Inside there is a treasure chest that contains the Square Jewel. In a regular playthrough of Oracle of Seasons, this treasure chest holds 100 Rupees instead of the Jewel.