Goddess Cube

Goddess Cubes are ornate metallic cubes scattered across the Surface in Skyward Sword. Each cube is the key to a specific one of the chests that the goddess Hylia left throughout The Sky.

Uses
In the Deep Woods, a Goron traveler named Gorko tells Link that he is researching about mysterious metallic cubes and that his ancient texts say for the hero to use a Skyward Strike. When Link uses the Skyward Strike on a Goddess Cube, it will transform into light and shoot into the sky.

These beams unlock Goddess Cube Chests scattered throughout the sky, which often contain treasures more valuable than those in regular chests, such as Pieces of Heart, Gold Rupees and Medals. Activated chests appear light purple in color; however, chests that are not activated appear dark gray in color and cannot be opened. Once Link returns to The Sky after activating a Goddess Cube, Fi will inform Link that it can detect an energy reaction given off by chests in the sky and will mark the locations of activated chests on his map.

Trivia

 * According to Gorko, both the Goddess Cubes and the Goddess Cube Chests are made out of an unknown metal.