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. Until the release of more crafting recipes such as 'lanterns', 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.

It has been found that Lightstone does not need a tool to be harvested.