Salt. Polarised light micrograph of a crystal of sodium chloride, the common salt. This is a white crystalline compound widespread in nature as a solid (rock salt), around margins of salt lakes and in solution (about 3%) in seawater. Other metals are present in the raw material such as calcium, iron and magnesium. These are often removed by precipitation before the salt is sold. Salt is used as a food seasoning and preservative, and as a raw material in industrial processes such as in glass and soap making. Magnification: x39 at 6x7cm size. Magnification:x66 at 5x6 inch size.