Vajra Deha is an unusual type of yogic achievement mentioned in the Yogasutra. Vajra deha is a human body, which has attained the strength of a diamond.
The human body is made out of Panchabhutas or five elements. When the earth element responsible of hardness attains special dominance, the body can attain the hardness of a diamond.
The belief is that such a body becomes so strong that no weapon made of any metal can harm it.
Sage Dadhici is believed to have attained such a body by the power of his penance.
It is also believed that in Ayurveda there are specific medicines that can give a person Vajra Deha.