Glowstone

Glowstone, formerly known as lightstone, is a golden block that glows indefinitely. This block can only be found naturally in the Nether. It generates on the undersides of Netherrack, and forms unusual coral-like stalactites. Glowstone has a luminance of 15.

Glowstone drops 2-4 pieces of glowstone dust, which may be used to re-craft the glowstone blocks or to boost the effect of a potion. Unlike torches, glowstone is not destroyed by water and continues to emit light even when submerged like Jack-O-Lanterns. Like all non-sunlight light sources, glowstone blocks will cause nearby snow and ice to melt.

It is more efficient to harvest glowstone blocks using Silk Touch Enchantment or Fortune Enchantment, since one block is equivalent to four glowstone Dust, while usual harvesting is more likely to drop less than mentioned. Note however, that when hitting glowstone with a pickaxe enchanted with fortune, glowstone still only drops up to 4 glowstone dust.

Harvesting Methods
Glowstone can be difficult to harvest, due to high ceilings, sheer cliffs, an abundance of lava in the Nether, and other hazards. While it's theoretically possible to mine high deposits by pillaring up and perhaps building a platform atop the pillar, this is likely to end badly as soon as a Ghast notices the miner. It's much safer to look for places where glowstone appears beneath a low ceiling, within easy mining distance of the ground.

With the addition of witches and witch huts in 1.4, glowstone blocks are able to be farmed infinitely. However, witch farming can often be fatal due to the power of the potions the witch uses to attack the player. Also, witches are not always guaranteed to drop glowstone.

Another way to obtain glowstone is to find a priest villager who will trade with you. Although this method can sometimes be rather expensive (and difficult if no villagers are offering it), it cuts out the dangers and hassle of going through the Nether or hunting down witches.

Glowstone can also be collected by redirecting a Ghast's fire ball at a glowstone deposit, destroying the blocks with the resulting explosion. This could be useful if the glowstone is very hard to reach.

Uses
Glowstone has a higher light level than torches (15 as opposed to 14). Because it is a block, players often make ceiling lights and lamp posts out of glowstone by combining it with a fence block. Glowstone blocks can also be used as lit paths, due to their interesting texture, or in docks or harbors so players in boats can see where to stop, since it can be placed in water. Also, glowstone can be used as a source of light for growing crops at night. However, you cannot place anything on glowstone except blocks and redstone, meaning torches, doors, etc. cannot be placed on it.

When placing redstone on a glowstone block, being a transparent block signals can pass from one block to diagonally another with glowstone "blocking but not blocking" it. Redstone wire, even though it may appear to connect up the side of a glowstone block and to the wire portion on top, will not send a redstone signal down the glowstone block. This allows for one-way signals without a repeater. This makes glowstone effective for vertical diodes, logic gate designs and space-efficient instant vertical redstone.

Trivia

 * Since glowstone blocks become entities when pushed by pistons, this momentarily stops it from giving off light.
 * Rarely, glowstone can be found touching the ground if the ceiling it was generated on was extremely low (2-3 blocks of clearance).
 * When glowstone is placed 9 or more blocks above ground level (out of its lighting range), glowstone will actually cast a shadow, despite it being a light-emitting block.
 * When you break glowstone, it has the same sound effect as breaking glass.
 * In the Xbox 360 Edition, because of the lack of glowstone clusters due to small worlds, Blazes drop 0-2 glowstone dust for a renewable source of glowstone.
 * Oddly, if a piston contraption is used to move a glowstone block onto a player's head, it will give an x-ray effect. This is useful should one want to find cave systems (though will kill you if done in survival without a partner).

Gallery
Glowstone (Block) Glowstone/es Glowstone (bloc) 발광석 Lichtsteen (blok) Jasnogłaz Светящийся камень 萤石块