Glass is currently the only transparent block that exists in-game. It is most commonly used for windows in houses, as well as for creating underwater structures. You can obtain glass by smelting sand. Currently, when breaking glass, no item is given in return, as you "break" the glass in-game. Six Glass can be crafted into sixteen glass panes. Three Glass can be crafted into three Glass Bottles which are used for Brewing. Furthermore, five Glass, three Obsidian, and one Nether Star will craft into one Beacon which is used as a light source, and is used to give the player special abilities. In the Redstone Update, also known as 1.5 update, three Glass, three Nether Quartz, and three Wooden Slabs will be used to craft a single Daylight Sensor, a new item being introduced to game that helps with Redstone contraptions. Mobs are unable to spawn on Glass, and Beacon light is able to travel through Glass. Glass blocks can be mined with the use of Silk Touch. All mobs other than Zombies, Slimes, and Spiders cannot see the player through Glass. Snow is unable to form on Glass. Thrown Items will be trapped inside Glass if the Glass is above said item.
Glass has 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.