Kala Namak
Kala namak (Urdu / Hindi); literally "black salt" is a type of rock salt, a salty and pungent-smelling condiment used in South Asia. It is also known as "Himalayan black salt", Sulemani namak, bit lobon, kala noon, or pada loon. It is found mostly in the Himalayas of Pakistan.
The condiment is composed largely of sodium chloride with several other components lending the salt its colour and smell. The smell is mainly due to its sulfur content. Because of the presences of Greigite (Fe3S4, Iron(II,III) sulfide) in the mineral, it forms brownish pink to dark violet translucent crystals when whole. When ground into a powder, its color ranges from purple to pink.
Kala Namak has been highly praised in India's Ayurveda. It has been used for its perceived medical qualities.
The raw material for producing kala namak was originally obtained from natural halite from mines in Bangladesh, India, Nepal and Pakistan in certain locations of the Himalayas salt ranges, or from salt harvested from the North Indian salt lakes of Sambhar Salt Lake or Didwana and the Mustang District of Nepal.
Traditionally, the salt was transformed from its raw natural forms into commercially sold kala namak through a reductive chemical process that transforms some of the naturally occurring sodium sulfate of the raw salt into pungent hydrogen sulfide and sodium sulfide. This involves firing the raw salts in a furnace for 24 hours while sealed in a ceramic jar with charcoal along with small quantities of harad seeds, amla, bahera, babul bark, or natron. The fired salt is then cooled, stored, and aged prior to sale. Kala namak is prepared in this manner in northern India with production concentrated in Hisar district, Haryana. The salt crystals appear black and are usually ground to a fine powder that is pink.
...
Wikipedia