Optical calcite is a natural mineral that's best known for its double refraction and this unique property has helped bring about a better understanding of the nature of light. A transparent variety of calcite which was originally discovered in Iceland, optical calcite is also known as Iceland spar although the name is not as common or as well known.
Optical calcite is used for the polarization of light and whilst I considered trying to explain this in more detail, I decided against it for fear of getting far too involved in the complicated subject of physics. What is quite interesting however is that optical calcite was used by the vikings for its light-polarizing properties to ascertain the direction of the sun on cloudy days for the purpose of navigation.
Some wonderful rainbows of colour can be seen in this piece of optical calcite and as well as being a fascinating mineral, it's also useful as a teaching aid for children in order to explain the nature of light. The term double refraction refers to the splitting of a single unpolarized ray of light into two separate rays the end result of which is basically like double vision.
Somewhat unsurprisingly, in crystal healing optical calcite is said to heal the eyes and can enable one to see double meanings behind words. It is also believed to reduce the tension that's associated with a migraine.
Weight (grams): 38.1
Size (cms): 4.5 x 2.7 x 1.3