This is because the speed of light in a material is inversely proportional to its refractive index. Diamond has the highest refractive index of any naturally occurring material, meaning it bends light the most.

Here's a breakdown:

* Speed of Light in a Vacuum: This is the fastest light can travel, approximately 299,792,458 meters per second.

* Refractive Index: This measures how much a material slows down light compared to its speed in a vacuum. A higher refractive index means light travels slower through the material.

So, because diamond has a very high refractive index (around 2.42), light travels significantly slower through it compared to other materials.