Glass

Glass is an almost completely transparent block which is very useful for windows and transparent floors/ceilings. It was added in version 0.0.19a. It is not affected by gravity, it is solid and allows light to pass through it.

Glass is created by placing a sand block in a Furnace

When broken, glass is lost forever and you do not receive a block of glass or sand to rebuild with. This means that glass is limited and should be used wisely. Also Using a shovel on glass will not gather it faster (even though it was sand). This is odd seeing as every other crafted material can be gathered faster with the original component's tool (Axe, Pickaxe)