Nonrenewable means that it cannot be replaced immediately or at all. Examples would be fossil fuels like oil and gas. It decreases as the population increases because it can be depleted. It is not readily available in abundance which is the reason why the world is putting great effort into finding renewable sources of energy.
If you are looking for what best describes nonrenewable resources, then the best statement from what you presented would be:
forms slowly that it may take millions of years to accumulate significant deposits.