It is a Alum Crystal.
This stone is used as a natural deodorant. It is a efficient alternative to traditionnal deodorant. Some block sweating and can notably cause breast cancer.
In China Alum Crystal is sold 5 mao and used as a deodorant...
In France this product invaded organic supermarkets and traditionnal supermarkets. It is sold as a miracle stone 12 euros... another great finding from China.