Glowstone

Lightstone (aka Brittle Gold, Glowstone, Australium) is a golden block that glows indefinitely. This block can be found in The Nether. It grows on the undersides of blocks and forms unusual coral-like stalactites. Lightstone has a luminance of 15, which is the same as sunlight and one more than torches. When breaking this block you get Lightstone Dust. Filling up a crafting bench grid with this yellow dust will create a placeable Lightstone block. Unlike other blocks formed in this way, a Lightstone block will not produce 9 pieces of Lightstone Dust when placed back in a crafting bench. Lightstone has no crafting use; however, it does have a practical use. Unlike torches, it isn't destroyed by water, and continues to emit light even when submerged. In this respect, it's a functional and easily applied underwater light. The block itself will actually melt nearby snow and ice.

It has been found that Lightstone does not need a tool to be harvested, and when destroyed uses the same sound glass makes when shattered. Using a sword will harvest the material much faster.

Lightstone is a potential item requirement to craft a lantern, as it provides the only infinite source of light in the game. This would also make lanterns relatively inaccessible and a risky item to get, as has been hinted at by Notch.