A natural resource is valuable due to its worth to the people. If it holds a worth, it would be in demand, and hence it would be valuable.
Also, some natural resources are not directly demanded by the people but they are still valuable. For example, some of the resources are used as substitutes or alternates to other resources and provide us with the same end result. In this case the natural resource has a value as well, may be not directly due to the resource itself but due to its use.