How many years marriage does an 'Emerald Anniversary' celebrate?

The 55th wedding anniversary is celebrated as the Emerald Anniversary.

The names of some anniversaries provide guidance for appropriate or traditional gifts for the spouses to give each other. These gifts vary in different countries, but some years have well-established connections now common to most nations: 5th Wood, 10th Tin, 15th Crystal, 20th China, 25th Silver, 30th Pearl, 35th Jade, 40th Ruby, 45th Sapphire, 50th Gold, 60th Diamond, and 70th Platinum. In English speaking countries the first, wooden, gift was cut on the day of celebration and then presented to the wife as a finished article before the next two quarter days had passed. The modern tradition may have originated in medieval Germany where, if a married couple lived to celebrate the 25th anniversary of their wedding, the wife was presented by her friends and neighbours with a silver wreath to congratulate them for the good fortune that had prolonged the lives of the couple for so many years. On celebration of the 50th, the wife received a wreath of gold.

Before 1937, only the 1st, 5th, 10th, 15th, 20th, 25th, 50th, and 75th anniversaries had an associated gift. In 1937, the American National Retail Jeweler Association (now known as Jewelers of America as a result of an organizational merger) introduced an expanded list of gifts. The revamped list gave a gift for each year up to the 25th, and then for every fifth anniversary after that.