The Short Version
This is a widely debunked myth. Natural diamonds form deep in Earth's mantle (150–200 km down) from carbon under extreme heat and pressure—not from coal. Most diamonds are billions of years older than land plants, which are coal's source material. Coal is a near-surface crustal rock not found at mantle depths. While a speculative, marginal possibility exists that subducted organic material could contribute to a tiny fraction of diamonds, this does not validate the claim as commonly understood. The scientific consensus is clear: diamonds do not come from compressed coal.