The most convincing evidence that coal did not play a role in the formation of most diamonds is a comparison between the age of Earth’s diamonds and the age of the earliest land plants.

Most of the rock-hosted diamond deposits that have been found were formed during the Precambrian Eon – the span of time between Earth’s formation (about 4,600 million years ago) and the start of the Cambrian Period (about 542 million years ago). In contrast, the earliest land plants did not appear on Earth until about 450 million years ago – nearly 100 million years after the formation of the vast majority of diamonds that have been mined.

Since coal is formed from terrestrial plant debris, and the oldest land plants are younger than almost every diamond that has ever been dated, it is easy to conclude that coal did not play a significant role in the formation of Earth’s natural diamonds.