This is a high clarity example of Iceland Spar in Polished Rhomboid form specifically because calcite, alongside its many variations sometimes grows naturally as Rhomboids.
Calcite is a very common and widespread mineral with highly variable forms and colors. Found in most geologic settings, Calcite as a later forming replacement mineral in most other environments in one form or another, it is most common as massive material in limestone's and marbles.
Iceland spar is a clear, transparent, colorless crystallized variety of calcite (calcium carbonate, CaCO3). Large pieces are split along natural cleavage planes to form natural rhombs. Iceland spar is probably best known for exhibiting the optical property of double refraction – so, anything viewed through the crystal appears double. It has many uses, in everything from precision optical instruments to LCD screens, and was even used during WWII to make bombsights. The perfect, flawless, colorless Iceland spar that is used in optical instruments sells for more than $1000 per kg.
Historically, the first, best quality, and most abundant source of this clear calcite was in Iceland, which is where it got its name. Recently, Iceland spar has been in the news because of new research that shows that this mineral was almost certainly used by the Vikings for navigation.
WHAT IS DOUBLE REFRACTION (BIREFERGENCE)?
The scientific term for double refraction is birefergence. This phenomenon occurs when a ray of light enters Iceland spar, and is split into two beams of polarized light, creating two images. Each image corresponds to one of the rays of light which has been split off and polarized. The light rays travel at different speeds, and are bent into two different angles (known as angles of refraction). When you look through Iceland spar, you see two images of everything.
A good way to view double refraction is to put a piece of Iceland spar on top of a piece of paper with a single straight line; looking inside the piece, you will see two lines. The thickness and angle of rotation of the Iceland spar determines the distance between the lines. If you rotate the piece to the right angle, the two lines will merge into a single line.