In the past women would keep the engagement ring even after they divorced. Today they are often sold, sometimes for financial reasons and sometimes to destroy any trace of the marriage's collapse.
In general, from a halachic point of view, it is not the man's duty to purchase an engagement ring for his fiancee, but only a commandment to give a modest wedding ring as part of the chupah ceremony. In the beginning of the 20th century, after the great depression in the USA, the diamond corporation 'De Beers' controlled the entire diamond trade in the world.