Red Ice

"Ice that glowed red possessed magical properties. Heat could not melt it, but the supernatural flame of Blue Fire could."

- The Legend of Zelda: Ocarina of Time Official Nintendo Player's Guide (Nintendo Power)

Red Ice is a peculiar type of ice found only in Ocarina of Time in Link's adulthood in Zora's Domain, the Ice Cavern, and Ganon's Castle.

Link's first encounter with the substance is upon his return to Zora's Domain seven years into the future: King Zora, as a product of the freezing of Zora's Domain, is trapped cold within a block of this Red Ice. Blue Fire is the only remedy for relinquishing King Zora from his icy red cage, and for dealing with any and all Red Ice found throughout the rest of Link's quest; Blue Fire can be bought for 300 Rupees at some shops (Empty Bottle required), or found in the Ice Cavern and Ganon's Castle. Many objects in the Ice Cavern are encased in Red Ice, such as Treasure Chests, Rupees, Jars, and even a Piece of Heart, which must be melted down in order to access.

Theory
Unlike regular ice, Red Ice does not freeze Link to the touch, suggesting that the ice is in fact "hot". This would appear to be supported by the fact that the ice can only be "melted" by the "cold flame" of Blue Fire.

Trivia

 * Red Ice may be based off of the crystallization of supersaturated sodium acetate solutions, also known as "hot ice".