Photo of optical calcite

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Optical calcite

These are naturally formed cleavage rhombs of optical calcite, known as Iceland spar. The mineral has three sets of cleavages - parallel planes of weakness - along which the stone will fracture under pressure. 

These are from Nuevo Leon in Mexico and and are clear enough to show the birefringence effect. Draw a cross on some paper, place the rhomb on top, and watch as you rotate it. There will be a double image that will spin round slightly differently.