Glowstone

"Glowstone was added to Minecraft on 30 October 2010 in the Halloween Update - which also added Jack o’Lanterns, clocks and fishing. It occurs naturally in the Nether, generating on the underside of netherack, but if you’re having trouble finding it then clerics in Minecraft villages will sometimes sell small amounts in exchange for emeralds.

Unless you’ve got a silk touch enchant on your tool, breaking a block of glowstone will shower you with 2-4 glowstone dust, which can then be combined in a 2x2 pattern in a crafting grid to make the block again."

- Duncan Geere

Glowstone is a light-emitting block that appears in the Nether.

Obtaining
Unless broken with a tool that is enchanted with Silk Touch or Fortune, a glowstone block drops 2–4 glowstone dust. If tool with Silk Touch is used, the block drops itself, and if a tool with Fortune is used, it has a higher chance of dropping a large amount of glowstone dust—but never more than 4.

Natural generation


Glowstone only occurs naturally in the Nether, where it generates in coral-like clusters on the undersides of netherrack.

Trading
Villager clerics will sell 1-3 glowstone for an emerald as a third tier trade.

Light source
Glowstone blocks emit a light level of 15, the brightest possible light level in the game. They emit the same amount of light as sea lanterns, jack o'lanterns, redstone lamps, conduits and beacons.

In redstone circuits
When placing redstone on a glowstone block, being a transparent block, signals can pass from one block to another diagonally. 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 makes glowstone effective for vertical diodes, logic gate designs and space-efficient instant vertical redstone.

Note that upside-down slabs can be used for the same purpose, and are easier to obtain.

Trivia

 * Since glowstone blocks become block entities when moved by pistons, this momentarily stops them 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), the glowstone will actually cast a shadow, despite being a light-emitting block.