The Question:

What gift or color signifies a tenth anniversary? (For example, the color representing the 25th anniversary is silver.)

The Answer:

There are several wedding anniversary customs. On the traditional list, the 10th anniversary is symbolized by tin. A more modern list assigns diamond jewelry to it.

(Not to be overly cynical, but the modern list is endorsed by several jewelry and gemstone associations, and through sheer coincidence contains nothing but jewelry and gemstones.)

—The Fact Monster