Molten Shard

The molten shard is a large lava rock in that falls from the top of Death Mountain due to Fyrus. It has two uses through the game.

Function
The molten shard falls at first when Link seeks access to the Goron Mines, after the twilight is removed from Eldin Province and Link is able to confront Gorons with the help of the Iron Boots. The shard is later used by Midna and Link, who take the advantage of the fragment's heat to melt all the ice away and thus free the frozen Zoras of Zora's Domain by warping the Shard there.

After the Molten Shard has been used to free the Zoras, and the Twilight has been lifted from the Lanayru Province, the Shard can be blown open using a Water Bomb to reveal a trapped Goron (who apparently fell into the lava in Death Mountain and was blown violently out of the volcano while inside the rock). Once freed, the Goron will give Link a Bomb Bag.

Trivia

 * The Goron trapped in the Molten Shard not only survived being trapped in a superheated shard of rock, but also the eruption, subsequent landing, and finally wandering around underwater for at least several days with no source of fresh air. They are certainly a hardy species, and confirms that (despite being poor swimmers) they're able to survive underwater.