Black rock salt, also known as Kala Namak or Himalayan black salt, is a kiln-fired rock salt with a distinctive pungent, sulfurous taste and aroma, used in South Asian cuisine and traditionally in Ayurvedic medicine. 
Here's a more detailed description:
Origin and Appearance:
Source:
It's mined in areas around the Himalayas, particularly in Nepal and Pakistan. 
Color:
The salt's color ranges from pinkish-brown to purple or dark violet, hence the name "black" salt, which is a misnomer. 
Processing:
It's a kiln-fired rock salt, meaning it undergoes a heating process that alters its composition and gives it its characteristic flavor and aroma. 
Taste and Aroma:
Unique Flavor: Kala Namak has a unique umami-like flavor, with a distinctive pungent, sulfurous taste and aroma, similar to a boiled egg.
Sulphurous Smell: The sulfur content is responsible for the characteristic pungent smell. 
Composition and Nutritional Value:
Sodium Chloride: It primarily consists of sodium chloride, like regular table salt. 
Trace Minerals: It contains trace amounts of sulfur compounds, iron, and other minerals, contributing to its distinct flavor and potential health benefits. 
Ayurvedic Properties: Kala Namak is considered a "cooling" spice in Ayurveda and is used as a digestive aid and laxative. 
Uses:
Culinary Applications:
It's used in South Asian cuisine as a seasoning, particularly in salads, chutneys, curries, and buttermilk. 
Vegan Substitute:
Some vegans use it as a substitute for eggs due to its sulfurous taste and aroma. 
Health Benefits:
It's believed to aid digestion, relieve gas and bloating, and potentially have other health benefits, though more research is needed to support these claims. 
Other uses:
It can be used to enhance the flavor of dishes, and some people use it as a digestive aid and laxative.