Cloud

A building block that can be found holding up a revamped floating island along with the Rain Cloud block. Light passes through Cloud as if it was empty space.

Cloud is unknown to be craftable or not, but can be harvested from multiple floating islands. If you were to make a base using cloud blocks, you would require a number of floating islands in order to harvest the cloud block from, as well as a large amount of gravitational potions in order to find the floating islands themselves.

Obtaining Gravitational potions
Gravitational potions can be found as a loot, but can also be obtained by downloading all item maps, such as the following, providing you with almost every item in the game: http://www.terrariaonline.com/threads/base-store-all-items-map-always-updated.33884/