Beach at Makauwahi Cave, Kauai, Hawaii

These are some of the grains collected from a strandline on the beach near Makauwahi Cave. When this sand is passed through a #100 US sieve the magnetite crystals dominate but this field includes a calcite crystal from the local limestone deposits. The bands of interference colors on the calcite crystal at the center of the field show the orientation of the crystal. The acute angle shown indicates that the low refractive index diretion in this crystal is north/south. The plane containing the carbonate ion is east/west. (Click here for more information on calcite.)