X-Shaped Jewel

The X-Shaped Jewel is one of the four Jewels needed to open Tarm Ruins. It is hidden on a island off the Western Coast. To reach it, Link had to use the Slingshot to light an offshore torch and create a bridge, and then drop a Mystery Seed onto a question mark, and then defeat the "Black Beast" (a Mini Moldorm). After doing that, a chest appeared with the X-Shaped Jewel inside.