Natural resources require the material to be found in nature and usable.
There are two criteria that allow a material to 'become' a natural resrouce. A natural process affects the material to allow it to be in a usable form. Manmade technology allows for the possibly of exploitation in an economical manner.
Two examples of natural process:
1 Small trees that are not harvestable but will grow into harvestable timber.
2 Organic remains that are allowed to form fossil fuels.
Resrouces that are not obtainable under current technology or economics:
1 Oil that is too deep or in a structure that current technology cannot extract cheap enough. If it costs 6 dollars to extract a gallon but it sells for 2 dollars then no one will mine it just to lose 4 dollars per gallon. (those numbers are complete fabricated for point of the illustration)
2 Also Uranium deposits that were not concentrated were not considered resources at the time of early Uranium mining, but solution mining now allows them to be considered natural resources.
EDIT:
A third criteria is a new use. If you had wheat growing wild but never ate it then it would not be a natural resource. Once someone developed a use for it then it would be a natural resource and the greatest thing before sliced bread!