Glowstone

Lightstone (aka Brittle Gold, 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. It gives off a soft light. (Gives off level 15 light) 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; Jack-o-lanterns do NOT provide more light than lightstone. Lightstone emits light not very far. 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.

Lightstone is a potential item requirement to craft lanterns, 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.