Why Are Diamonds A Non-renewable Resource?

Diamonds form extremely slowly. It takes over one billion years for a diamond to be created by the heat and pressure of the Earth.  This means that once all the diamonds which exist right now are mined we will have to wait billions of years before doing any more diamond mining, and that is a very very long time.

Diamonds are renewable because volcanoes are always erupting

Diamonds take billions of year to make, slower than its demand.
Also, artificial diamonds also cannot meet the demand.