This is a special piece of Calcite known as Golden Calcite, due to its rich yellow color. This type of Calcite, unlike the more common opaque varieties has optical properties and displays many rainbows and shimmering plays of color. this piece has been carved into a Rhomboid shape specifically because calcite, alongside its many variations sometimes grows naturally as Rhomboids.
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.
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.