This calcite is from Corumba in Mato Grosso dul Brazil, Brazil. The calcite from this locale is also called Iceland Spar. Iceland Spar is named after the location in which this optical type of calcite was originally found in the Helgustadir Mine, Eskifjor, Iceland It is highly transparent and has a well defined birefringence (Double Vision). This piece has been cleaved along its natural cleavage planes which are parallel at 30 and 60 degree angles. This results in a sharp natural unpolished rhobohedral shape. This piece is highly transparent colorless and has a well defined birefringence or double vision quality. It measures 2 3/4 x 1 5/8 x 3/4 inches or 70 x 41 x 20 mm as measured at right angles to the cleavage planes.