Diamond Quality Pyramid makes it easy to understand the link between the 4Cs and diamond pricing. The link is rarity. This is what makes one diamond more expensive than another.

Imagine that all of the world's diamonds form a pyramid. While all diamonds are rare, those that are available in greater numbers fill the large base of the pyramid. Those diamonds closer to the top are fewer in number. The tip of the pyramid represents the rarest of the rare. These diamonds are fewest and so they are
more valuable and more expensive.