The Crystal Chest is transparent, so you can see the items that are in it from the outside. It is compatible with Pipes and Pneumatic Tubes. The Crystal Chest requires a Stone Pickaxe or greater to be collected. It has the same amount of storage slots as the Diamond Chest, but is transparent. There is no benefit to making a Crystal Chest other than for looks. It is from the Iron Chests Mod.
Warning: This chest may cause lag on slower computers and servers: Reference
Some items appear to have different colors inside the chest, including grass.