Diamonds themselves do not emit light, but they have strong light absorption. When a diamond is cut by hand, it can concentrate the incident light from the outside and reflect it from one surface. This is a physical principle of light and reflection. Light is swaying, that is, it constantly changes the path of light incident on the diamond, which also changes the path of light reflected by the diamond, so it can have a sparkling effect.

The following picture shows the reflection of perfect cut diamond light:

Image source: Luxury Jewelry Diamond Network, a website specializing in diamond wholesale.

Reflection diagram of imperfect cut diamond;