Diamonds do not contain a significant amount of carbon-14. Carbon-14 is a radioactive isotope of carbon that is present in trace amounts in the Earth's atmosphere. It is formed through interactions between cosmic rays and atmospheric nitrogen.
Diamonds, on the other hand, are formed deep within the Earth's mantle under intense heat and pressure. The carbon atoms that make up diamonds are typically derived from ancient carbon sources, such as organic matter that has been subjected to geological processes over millions of years.
Since diamonds are formed over such long timescales and deep within the Earth, any carbon-14 that may have originally been present in the organic material has long since decayed. The half-life of carbon-14 is approximately 5,730 years, meaning that after several tens of thousands of years, the amount of carbon-14 remaining in a sample would be negligible.
Therefore, diamonds are not a significant source of carbon-14. The carbon in diamonds is predominantly the stable isotope carbon-12, along with a small fraction of carbon-13, which is another stable isotope of carbon.