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; however, Jack-O-Lanterns produce more light and also don't get destroyed underwater, so they are a better choice.

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.