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 either very 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. 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.

The glowstone block can be used underwater as a light source, making it an alternative to Jack-O-Lanterns, which can also be used underwater.

As of 1.6.4, any item such as rails, torches, buttons, levers, etc. will not be able to be built onto Glowstone. With the update, Glowstone now acts like glass. Anything that was previously on Glowstone will fall off, and will not be able to be rebuilt onto.

Trivia

 * 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.
 * Glowstone gives off the same amount of light as sunlight.