What is the difference between "scarce" and "rare"?

What is the difference between "scarce" and "rare"?

And how to say the following?

diamond is (rare, scarce)

My English teacher has said that there is a difference between the two words, later, I asked him for more explanation but I couldn't fully understand. He said that rare is to describe the valuable thing, for example --> ‘rare pearl’, and scarce for the thing being elusive and can't be easily obtained as there is not enough amount of it.

Solution 1:

Your teacher is somewhat right about the difference. Yet, @Mitch defines the words correctly in the comments.

• Rare is used for something valuable, but something that is not in demand or is not necessity of people.

According to Google:

"(of a thing) not found in large numbers and so of interest or value."

For example: Diamonds are rare.

• Scarce is used for something that is needed by people, is in high demand.

As Google describes it:

"(especially of food, money, or some other resource) insufficient for the demand."

For Example: Food became scarce in the town.