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 can be difficult to harvest, due to high ceilings, sheer cliffs, an abundance of lava in the Nether, and other hazards. One method of mining it is pillaring up to the deposit and extending the top of the column beneath it to make a platform. Dust will fall onto the platform, allowing for easy collection.

Another way to obtain glowstone is to find a librarian 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.

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.

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 it can send power up, but not down the block, like upside-down half slabs, since the block is considered transparent. This makes glowstone effective for vertical diodes and space-efficient instant vertical redstone.

Bugs

 * Altering glowstone or rails on glowstone (from Beta 1.8 and before) appears to cause the game to crash. This seems to occur when the rails extend to outside of the render distance.
 * This has not been verified as a bug, but in both Single Player and Multiplayer, when you have glowstone pushed into the same block as your head, you can see through the ground. You will be able to see hollowed out areas, like dungeons, caves, ravines, and mine shafts. To do this you dig a hole two deep, and pick a direction and go two blocks in that direction, making a sideways "L". Next, you place some redstone wiring and repeaters, making the delay as long as possible. Activate the redstone and quickly go into the hole. The piston will push the glowstone into your head. You can see through the ground, but you won't suffocate. (Also works with TNT.)

Trivia

 * Another way of collecting glowstone is to redirect 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.
 * It is more efficient to harvest glowstone 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.
 * Redstone can be put on glowstone, but being a transparent block signals can pass from one block to diagonally another with glowstone "blocking but not blocking" it, making possible nearly instant vertical redstone and new vertical logic gate designs. 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.
 * 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 hitting glowstone with a pickaxe enchanted with fortune, glowstone still only drops up to 4 glowstone dust.
 * When you break glowstone, it has the same sound effect as breaking glass.
 * Glowstone blocks are able to be farmed infinitely now, since witch huts were added in 1.4.