Invisibility Cloak Hides Objects Visible To The Naked Eye - Technology Review: "Zhang and co realised that there are naturally occurring materials that can do the same thing. Calcite is one of them. It is unusual because its optical properties depend on the direction that light passes through it.
By carefully exploiting this property, they've been able to create a block of calcite (actually two blocks of calcite) that acts like a carpet cloak. They've even demonstrated it by hiding a wedge of steel 38mm long and 2 mm high. Zhang and co say that this is the first time that a visible object has ever been cloaked"