But, I think they were made of seashell, not silver.

The ancient Sumerians were the first to trade objects, or "currency", for other items of value. The Sumerians created "coinage" by taking cone shaped shells and cutting slices into them, which created rings.

The shell coin is much sturdier than you might think, is fairly heavy, and stronger than its delicate appearance suggests. Consistent with the practice of the ancient Sumerians, one could wear the shell as a pendant on a chain.

Source: Ancient Gifts