Final answer:
Resources are considered rare when they have no structural equivalent. This means that a resource is rare when there are no other resources that can be substituted for it.
Step-by-step explanation:
Resources are considered rare when they have no structural equivalent. This means that a resource is rare when there are no other resources that can be substituted for it.
For example, if a certain type of mineral deposit is the only source of a particular metal, and there are no other minerals that can be used to produce that metal, then the resource is considered rare.
On the other hand, if there are multiple minerals that can be used to produce the same metal, then the resource is not considered rare.
It's important to note that rarity does not necessarily imply value.
A resource can be rare but not valuable if there is no demand for it or if it is difficult to extract or use effectively. Likewise, a resource can be abundant but valuable if there is high demand for it or if it has unique properties or uses.