Why is a diamond a non renewable resource?

Non-renewable. A diamond is pure carbon. It takes very high heat, but it will burn and leave no ash. That is the energy answer.

If I understand your question, it is non-renewable. Regardless of how a diamond is formed, naturally or artificially, it is made of carbon. You are taking material from the earth and using it to make a diamond. You are not putting anything back to be recycled.