A Glowstone Block is a naturally occcuring block found often in the Nether. It is often found in lumps hanging from the ceiling. When destroyed with any pickaxe it will drop 1-4 Glowstone Dust. Glowstone Blocks can be crafted with 4 glowstone dust in a 2x2 square.
Glowstone can be used as lighting rather than torches (also has a higher light intensity than a torch). Glowstone can be placed underwater much like Jack-O-Lanterns. Glowstone can also be used along with Redstone to make Redstone Lamps. An advantage the Glowstone Block has over the torch is that it can used as a ceiling for your room, whereas torches can only be placed on walls.
Blocks with Opacity cannot have all standard blocks placed on them. Blocks with Opacity are as follows: Water (Source), Lava (Source), Tall Grass, Oak Sapling, Birch Sapling, Spruce Sapling, Jungle Sapling, Red Mushroom, Brown Mushroom, Red Flower, Dandelion, Crops, Sugar Cane, Cactus, End Portal, Portal, Fire, Sign, Torch, Snow, Air, Enchantment Table, Redstone Torch, Redstone Wire, Button, Pressure Plate, Lever, Rail, Trapdoor, Redstone Repeater, Door, Iron Door, Bed, Cake, Fence, Ladder, Oak Wood Stairs, Spruce Wood Stairs, Jungle Wood Stairs, Birch Wood Stairs, Cobblestone Stairs, Bricks Stairs, Stone Brick Stairs, Nether Brick Stairs, Sandstone Stairs, Quartz Stairs, Stone Slab, Sandstone Slab, Cobblestone Slab, Bricks Slab, Stone Brick Slab, Oak Wood Slab, Jungle Wood Slab, Birch Wood Slab, Spruce Wood Slab, Nether Brick Slab, Quartz Slab, Farmland, Chest, Large Chest, Glowstone, Piston, Sticky Piston, Oak Leaves, Jungle Leaves, Spruce Leaves, Birch Leaves, Glass, and Ice.