Okay kiddo, have you ever seen a diamond? It's a very pretty and shiny rock!
Now, the people who work with diamonds and sell them are called diamantaires. They are like the special teachers of diamonds. They know everything there is to know about diamonds and how to make them even more beautiful.
When someone wants to buy a diamond, they go to a diamantaire to buy it because they know that the diamantaire will give them the best advice and help them choose the perfect diamond.
Diamantaires are important because they make sure that the diamonds are of the best quality and are not fake or damaged. They also make sure that the diamonds are sold at a fair price.
So, in short, diamantaires are special experts in diamonds who help people buy the perfect diamond and also make sure that everyone gets a good deal.