Kala Namak

Kala namak, or black salt, is a kiln-fired rock salt with a sulphurous, pungent smell, used mainly in South Asia and made from salts mined around the Himalayas. It is largely crystalline sodium chloride heated with charcoal and herbal components that give it its colour and smell, with traces of sodium sulphate, iron sulphide and hydrogen sulphide. Whole, it is brown and black, and ground to powder it ranges from pink to dark black-brown. Traditionally the raw salts were fired in a sealed ceramic jar for 24 hours with charcoal and small quantities of harad seeds, amla, bahera or babul bark. The page says it is now common to add the chemicals to pure salt before firing. It is a condiment in South Asian cooking, and Ayurveda considers it a cooling spice used as a laxative and digestive aid.