Jack o'Lantern

Jack 'o' Lanterns (or Jack-O-Lanterns) are solid blocks implemented in the Halloween Update on October 31st 2010. It can be created by combining a pumpkin and a torch. They are intended as a purely decorative addition although they can also be used as a light source.

As with the pumpkin, a Jack 'o' Lantern can be placed anywhere as long as the block directly beneath it is a solid, opaque block (except TNT); thus, they can be placed at the bottom of lakes or oceans, allowing the user to easily explore underwater areas. However, it will remain in place if you remove the block below it after placing it. They emit light in every direction and the face does not need to be visible to emit light, so it can be very useful in creating hidden lights.

Jack 'o' Lanterns produce the same amount of light (level 15) and share the same ability to stay lit underwater as glowstone, making them more effective and more reliable than torches. Jack 'o' Lanterns will also melt Snow and Ice.

When attempting to push a Jack 'o' Lantern with a piston, it will break and drop as a resource.

Jack 'o' Lanterns can also be used to make a Snow Golem and an Iron Golem, although golems made in this way are no different, aesthetically or functionally, from golems made with regular pumpkins.

Trivia

 * At Halloween (October 31st), Zombie Pigmen have a chance of spawning with a Jack 'o' Lantern equipped as headgear.
 * Jack 'o' Lanterns cannot be placed on Air, Fences, Glass, Flowers, Mushrooms, Pressure Plates, Sugar Cane, Torches, Redstone Torches, Redstone Dust, Signs, Glowstone, Ice, Pistons, any Stairs, Iron Bars, Slabs or Sticky Pistons.
 * In SMP, Jack 'o' Lanterns appear green on a map.
 * If a player removes the solid block beneath a Jack 'o' Lantern placed underwater, swimming directly beneath it and touching the Jack 'o' Lantern will cause the game to think the player is above water, allowing the player to refill on air without surfacing, also allowing the player to see what is underwater.
 * Jack 'o' Lanterns are useful to create cheap breadcrumb beacons in the nether, because unlike torches they will stay in place even if the netherrack it has been placed on is destroyed, and unless it is directly hit it can often resist ghast bombs explosions, unlike glowstone blocks. Also, it stands out more than glowstone beacons because it cannot be found naturally. It is also better than fire, because extended fights against ghasts will often leave the landscape torn apart and burning everywhere.
 * Despite presumably containing a torch as their light source, Jack 'o' Lanterns emit more light than torches.
 * Unlike pumpkins, Jack 'o' Lanterns cannot be worn on the player's head. However, Zombies, Zombie Villagers Zombie Pigmen, Skeletons, Skeletons on Spiders and Wither Skeletons have been observed rarely wearing Jack 'o' Lanterns on October 31 (Halloween) alongside pumpkins; the worn Jack 'o' Lanterns do not glow or emit light.
 * Jack 'o' Lanterns are the only self contained light source that minecart Rails can be placed on.
 * Jack 'o' Lanterns have what may be the most complex crafting chain that does not require a Crafting Table: Wood->Planks->Sticks, then add coal/charcoal and the pumpkin.