In English, an "ivory wedding anniversary" commemorates 30 years of marriage. Pure and milky white, ivory, like gold and diamonds, possesses a material character which doesn't change over time and is a symbol of a long marriage.
英文字裡以「象牙婚」讚譽牽手走過三十年的婚姻，就因為純潔乳白的象牙，與黃金、鑽石等貴重金屬是永不變質、白首偕老的象徵。