Colorless and clear rhombohedral variety of calcite exhibiting extremely marked birefringence. Used for manufacturing optical instruments. Minimum size: 2.5 x 2 x 1.5 cm. If you place the crystal above a dot, you will see 2 dots; if you rotate the crystal above the dot, you will then see one dot rotate around the other. This phenomenon is called the extraordinary ray; the other dot is called the ordinary ray.