Q:

stone which turns to black on exposing to atmosphere?

My mom is using a stone like substance as a dye for hair recommended by a herbalist. That stone becomes black when exposed to air. Do you know which compound is this?

A:

There is a compound called cerussite or 'natural white lead' which turns to black 'lead sulphide' when it reacts with hydrogen sulphide in the atmosphere. It was commonly used as make up by women in medieval times and is sometimes still used as a white pigment in 'lead paint'. 
Of course, we all now know lead is toxic to many organs and tissues including the heart, bones, intestines, kidneys, and reproductive and nervous systems, as well as causing potentially permanent learning and behavior disorders, particularly in children.
For this reason, it's use has diminished over time, except perhaps amongst New Age crystal 'healers' who claim it can "infuse your aura with light" and "bring amazing change into your life", not only through a lowering of general intelligence, but, it can also help prevent bacterial infections.
Having said all this, you may be surprised to know that another toxic lead-based white crystalline substance called lead acetate is approved by the FDA for use in 'progressive' hair dyes, where it reacts with the sulphur in hair proteins to slowly turn grey hair black, over several applications of course, so 'no-one' will notice.

image of lead(II) acetate crystals, courtesy of Wikipedia
PS: In case you were wondering why lead-based products are still used, given that there are many alternatives out there, many of which are labelled 'non-toxic', 'natural' and 'organic'...
the National Cancer Institute reports that there are over 5000 different chemicals used in hair dyes, including toxic coal-tars, aromatic amines and phenols. Some of these are also carcinogenic (cancer-causing agents) and can be absorbed through the skin or inhaled. Whilst the industry has been working towards eliminating some of these chemicals since the late 1970's, people who regularly began using hair dyes before 1980 may be at increased risk of developing non-Hodgkin lymphoma (a form of blood cancer).
The International Agency for Research on Cancer (IARC) has said that "whilst workplace exposure as a hairdresser or barber is probably carcinogenic to humans", personal hair dye use is “not classifiable as to its carcinogenicity to humans,” based on "a lack of evidence from studies in people".

