These Crystal Clear Calcite Cubes (often referred to as Optical Calcite or Iceland Spar) are renowned for their incredible transparency and perfect rhombohedral cleavage. While they appear as perfect cubes, they are actually slanted rhombohedrons that showcase the mineral's unique double refraction. When placed over text or a line, the crystal creates a clear double image, making it a fascinating "optical" specimen for any collection. Their glassy luster and high clarity make them look almost like blocks of solid ice.