White Sapphire and Diamond may appear same but if you observe them under a dichroscope, you will see white Sapphire are doubly refractive gems while diamond will have single refraction. However, a diamond may sparkle more than a White Sapphire. They both are scratch resistant but the hardness is not same. Diamond is thehardest stone having 10 Mohs whereas White Sapphire has 9 Mohs.
