Their effective abundance depends on the natural processes that make them available and the social processes which decide which are actually used. Natural materials can only be used if they are identified and if technology is capable of extracting them and transforming them into useful products. In the modern world, they tend to be used only if they are cheaper than alternatives, though politics can also influence whether or not a resource is used. Resources are categorized into two types. Stock resources are those which exist in finite quantities, usually as a result of geological concentration over very long timescales.
Resource depletion has different meanings for the two types.
The obvious examples of stock resources are metal ores and fossil fuels. In fact modern society uses much larger quantities of building stone or aggregate, but with rare exceptions such stone is of low value and is only transported short distances. The concept of an ore is tautologous because metals exist in almost all rocks, but only those with the highest concentrations of the most suitable chemical composition are regarded as ores. Different metals are exploited at very different concentrations. However, lower grade ores will be more expensive, will require more energy to refine and will produce larger voids and more spoil.