CHARACTERISTICS AND TESTING AS IN ANCIENT TEXTS

Among the stones and gems, diamond is the hardest, brightest, transparent and the most precious.
Varahamihira has termed the diamonds as Vajramani, and defined its characteristics as the jewel which is as hard as Vajra, that radiates rays when put into a shallow water and the rays float, which is as pure and genuine as electricity, which ensures happiness to all, like the rainbow and fire does naturally.
(cf: Brihat Samhita; Chapter 79; shlokas 1-5)