Wood is also a good exemple of the different values.
First, we all know the use value of wood. Wood can be use to build our furniture, our house, all kind of buildings, tools, tootpicks, newspaper, books, etc.
But what about the exchange value of the wood? Of the trees?
When the first Europeans arrived in America, they found trees everywhere. They cut them and now, we have large farms. The same happened in every country of the world: forest were cut by human to make large fields and to grow our food. Today, only a few forests remain and some great hearted people want to keep them, but would they accept to reforest their own large farms of Europe and North America?
What is the value of a tree?
When I was a college teacher, I gave the exemple of these small trees on the campus, that I was usually able to show through the classroom windows. They had less than 10 cm of diameter; so, I asked my students how much someone who would cut one of them would have to pay back to the college? A lot of money, many hundred of dollars for sure.
So, it would be better to go in a near forest, less than 10 kilometers for the college, and to cut a real big tree there, forty or fifty meters high with his chain saw. The owner of this forest is Domtar, a very big company. How much would have to pay the amateur woodcutter? The cost of the corporate lawyers is too expensive to care about that!
But in the Amazom, there are a lot of trees, and very big ones. How much would have to pay the same amateur woodcutter who just decides to drop there by parachute with his pro chain saw and some food? He wants to build his house in one of these trees and live there! Nobody will ever know that this crazy guy even went there!
So the value of one tree is not related to the amount of wood in it, which is its use value. On the college campus, what use value has the small tree? None, but a very important exchange value, because you need to plant another tree. For Domtar, one big tree provides a lot of raw material for its paper mill, but the cost of transportation is as much important; so the exchange value, taking in account the cost of the lawyers, is not relevant. Finally, that tree in the Amazon forest could had served to build many houses, but it has no exchange value; Brazilian will rather burn their forest because cutting the trees is too long and cost more.
I want to take the opportunity to invite you to see the video of Frederick Back "L'homme qui plantait des arbres", à partir du texte de Jean Giono: http://www.fredericback.com/cineaste/filmographie/lhomme-qui-plantait-des-arbres/index.fr.shtml;< you will find it on Youtube too.