Glowstone

Glowstone (formerly known as Lightstone, Brittle Gold, Brightstone, Australium or Brimstone) is a golden block that glows indefinitely. This block can only be found in The Nether. It grows on the undersides of blocks, commonly near lava, and forms unusual coral-like stalactites that may resemble frozen lightning. Glowstone has a luminance of 15.

Glowstone clusters can be found way up high, or at almost ground level. It may be located over lava or in extremely high places, making it hard to harvest.

Breaking this block drops Glowstone dust, which may be used to re-craft the glowstone blocks. Glowstone itself has no use in crafting; however, unlike torches, it isn't destroyed by water and continues to emit light even when submerged, allowing for a tasteful alternative to Jack-O-Lanterns for underwater construction. Like all non-sunlight light sources, Glowstone blocks will melt nearby snow and ice. This Block also can be used as a chandelier, like a Jack-O-Lantern.

Glowstone does not require a tool to be harvested, and makes the same sound as glass when shattered. Swords can be used to break Glowstone at a faster pace, with the cost of two uses.

Underwater Use
The glowstone block can be used underwater as a light source. The Jack-O-Lantern (from the Halloween update) can also be used underwater.

Trivia
So if it is placed at layer 127 in the normal world, its top surface brightness changes with the day-night cycle. If placed at layer 127 in the Nether, where there is no day-night cycle the top surface remains as dark as adjacent ordinary netherack or bedrock blocks. This effect is more difficult to see when the soft lighting setting is used because the luminance of the block is slightly at odds with the illuminance it causes on immediately adjacent blocks. It is suspected that the same occurs at layer 1, where the bottom surface of a glowstone block faces the void and may well also be true at the horizontal limit of a map but one could never see it in game of course, becauese as the avatar approaches, new chunks are loaded to move the edge further out.
 * Glowstone could potentially be an item requirement to craft a lantern, as it also provides an infinite source of light. This would also make lanterns relatively inaccessible and a risky item to get, as has been hinted at by Notch.
 * In both Alpha and Beta, if Glowstone is placed at the limit of the map, i.e. at layer 127, the sides of the block exhibit the usual luminance, but the top surface exhibits an illuminance which is dependent upon the ambient light level, the same as non luminous blocks.