The origins of the diamond, as well as its many connotations, lies in India where it was first mined. The word most generally used for diamond in Sanskrit is transliterated as vajra, "thunderbolt" and indrayudha or "Indra's weapon." As Indra is the warrior god of Vedic scriptures (the foundation of Hinduism, the symbol of the thunderbolt tells a lot about the Indian conception of diamonds. The flash of lightning reminds of the light that is reflected by a fine diamond octahedron and a diamond's indomitable hardness.
Related information:
1. What is the origin of the word 'diamond?'
2. What is the royal history of diamonds?