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.
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.