Calcite, Optical is a transparent variety of calcite, or crystallized calcium carbonate, originally brought from Iceland, and used in demonstrating the polarization of light, and is also referred to as Iceland Spar. 

Optical Calcite possesses several optical properties other than double refraction and birefringence.  It is highly transparent to visible light, allowing light to pass through with minimal absorption or scattering, which is ideal for optical applications requiring clarity.  It can produce vivid colors when viewed under polarized light due to its birefringent nature, this effect is known as the "Becke line" and can be used to determine a mineral's refractive index.  Additionally, optical calcite is optically active, meaning it can rotate the plane of polarization of light passing through it, a property resulting from its asymmetrical atomic arrangement.

If you pass the stone over a piece of paper with an image or text, you'll see birefringence.

Perhaps the most interesting modern research using optical calcite is to actually make things invisible.  Researchers have found that using a pair of shaped calcite crystals in certain media, they can effectively make small objects disappear in visible light. Light from behind the object bends right around it while passing through the two crystals.