Optical Calcite is also known as Iceland Spar, and was originally discovered in Iceland, hence its name. It is a transparent form of Calcite, which often displays beautiful rainbows that flash in the light. It also displays the rare property of double refraction, so when placed over text it will show double the image, slightly offset. This beautiful crystal is most often seen in a colourless form, but it also forms in pink, peach and green tinged crystals, often named Watermelon Calcite for their colours. Optical Calcite can also be found in Mexico and New Mexico, USA.