Here's what tradition says:

20th Anniversary Traditional Gift:
China. It symbolizes the beautiful, elegant, and delicate of your love for one another over the past 20 years.
20th Anniversary Contemporary/Modern Gift:
Platinum. Platinum is strong and enduring, just like your 20-year old marriage.
20th Anniversary Gemstone:
Emerald or a yellow or golden diamond.
20th Anniversary Color:
Emerald green or white.
20th Anniversary Flower:
Day Lily.