Goddess Cube

Goddess Cubes are mysterious cubes that are scattered all over The Surface in Skyward Sword.

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 disappear and a sphere of light will shoot into the sky.

These beams activate Goddess Cube Chests scattered throughout the sky, which often contain treasures that are 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.