Question

हीरे के चमकने का क्या कारण है​

Answer 1

Diamond has a very high refractive index (about 2.42 compared to about 1.5 for glass). The amount of light reflected at an air/whatever interface is related to the refractive index change at the interface, and generally speaking the bigger the refractive index change the more light is reflected. So if you compare a diamond to a piece of glass cut into the same shape, the diamond will reflect more light and therefore it will sparkle more.

The sparkling isn't just the high refractive index - the shape matters too. Diamonds are cut so that a light ray falling on the diamond is reflected multiple times inside the diamond and therefore sends light out in all directions