Не очень понимаю почему утверждение ложно. Был обычай дарить на помолвку кольцо. ДеБирс сумела создать обычай дарить кольцо непременно с бриллиантом, при этом вокруг бриллиантов появился ореол романтики и пр.

Там и про Японию тоже упоминают. Вот уж где не было никакого обычая ни в каком виде. А потом он появился. Вот маленькая цитата:

The campaign was remarkably successful. Until 1959, the importation of diamonds had not even been permitted by the postwar Japanese government. When the campaign began, in 1967, not quite 5 percent of engaged Japanese women received a diamond engagement ring. By 1972, the proportion had risen to 27 percent. By 1978, half of all Japanese women who were married wore a diamond; by 1981, some 60 percent of Japanese brides wore diamonds. In a mere fourteen years, the 1,500-year Japanese tradition had been radically revised. Diamonds became a staple of the Japanese marriage. Japan became the second largest market, after the United States, for the sale of diamond engagement rings.