>They never had any value

They had immense value in ancient world. They were valued much more than gold - due to their rarity. Southern India was the only known source for mining diamonds in ancient world [1]. They were used as currency and was valued higher than gold. In fact there were wars fought for diamonds.

There is a saying about Koh-i-Noor, one of the most famous Diamond - "If a strong man were to throw four stones – one north, one south, one east, one west, and a fifth stone up into the air – and if the space between them were to be filled with gold, all would not equal the value of the Koh-i-Noor" [2].

[1] https://en.wikipedia.org/wiki/Golconda_diamonds

[2] https://www.theguardian.com/commentisfree/2016/feb/16/koh-i-...