Fabric of Reality

Fabric of Reality is the primary block type in Pocket Dimensions and Dimensional Dungeons.

It absorbs all light and has no texture, it also emits light. When destroyed (by any means except with Silk Touch or IC2's Rock Cutter) it will not drop any blocks, so the only way to acquire Fabric of Reality is by finding stacks of it in Dimensional Dungeon chests. Any block placed on the fabric will replace the fabric entirely, but holding shift will allow blocks to be placed on the fabric without replacing it. Additionally, placing a transparent block on the Fabric of Reality will not destroy it, and the block will be placed without replacing it. It is from this that the Fabric of Reality is believed to be transparent.

While it is safe to walk on, you need to watch your step, when exploring Dimensional Dungeons, as its edges are just as black as the void below it and there are many traps and hidden treasure rooms that utilize this, holding shift will prevent you from falling.

Fabric of Reality (214)
There is also another Fabric of Reality block, with an ID of 214, that has the exact same properties but can have any block placed on it (without needing to hold shift).

It is only found forming the outer layer of a Pocket Dimension. It cannot be mined by hand, however, a RedPower Block Breaker, BuildCraft Quarry, or MineFactory Reloaded Block Breaker will break it and allow you to pick it up.