Redstone Torches are items the give off redstone signals. They can be powered and depowered through blocks.
Redstone Torches have opacity. 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.