NR. Natural Resources

NR: Natural Resources [It may be best to use linked notation for all subclasses here.]

1. Energy resources

n. Non-renewable (general)

nc. Coal

nn. Natural gas

no. Oil

nu. Nuclear

r. Renewable (general)

rg. Geothermal

rh. Hydro-electric

rs. Solar

rw. From waste products

2. Minerals [Specific minerals are addressed in R]

3. Water

4. Air

6. Biological [Biodiversity etc.] [Better to link to NF]