Halite, or rock salt is sodium chloride (NaCl).This is an evaporite mineral which has precipitated from evaporating sea water. The "dirty" colour is due to contamination by impurities. This specimen from a Cheshire salt mine requires extra care in storage due to it's tendency to decompose in humid conditions, and become very wet. This specimen is stored in an airtight box,with a good number of dessicant sachets, but even so is still liable to become slightly wet.