'Optical Calcite'

Mexico

Iceland Spar is an intriguing form of Calcite.

The crystals have formed into a beautiful double refractive shape.

This can be demonstrated by holding a crystal over some printed words
The letters you look at will be doubled and appear
I have shown this effect in one of the images on the listing.

A ray of unpolarized light passing through the crystal divides into two rays of perpendicular polarization directed at different angles, called double refraction.

So objects seen through the crystal appear doubled.

Historically, the double-refraction property of this crystal was important to understanding the nature of light as a wave. This was studied at length by Christiaan Huygensand Isaac Newton. Sir George Stokes also studied the phenomenon.

 Its complete explanation in terms of light polarization was published by Augustin-Jean Fresnel in the 1820s.

The crystal has three sets of cleavages - parallel planes of weakness - along which the stone will fracture under pressure. 

 You will receive the  crystal in the image -  measuring 2.8cm across


You will also receive an information card about Optical Calcite so you will always know WHY these are such magical objects.  ( image attached)